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The Gregorian solar calendar is an arithmetical calendar. It counts days as the basic unit of time, grouping them into years of 365 or 366 days. The solar calendar repeats completely every 146,097 days, which fill 400 years, and which also happens to be 20871 seven-day weeks. Of these 400 years, 303 (the "common years") have 365 days, and 97 - the leap years - have 366 days. This gives an average year length of exactly 365.2425 days - or 365 days, 5 hours, 49 minutes and 12 seconds.
A Gregorian year is divided into twelve months of irregular length (but note that there is a period of 153 days divided over 5 months in an alternating pattern from March to July that repeats from August to December):
|2||February||28 or 29|
A calendar date is fully specified by the year (numbered by some scheme beyond the scope of the calendar itself), the month (identified by name or number), and the day of the month (numbered sequentially starting at 1).
Leap years are all years divisible by 4, with the exception of those divisible by 100, but not by 400. These 366-day years add a 29th day to February, which normally has 28 days. Thus, the essential ongoing differential feature of the Gregorian calendar, as opposed to the Julian calendar, is that the Gregorian omits 3 leap days every 400 years. This difference would have been more noticeable in modern memory, were it not for the fact that the year 2000 was a leap year in both the Julian and Gregorian calendar systems.
The intercalary day in a leap year is known as a leap day. Since Roman times February 24 (bissextile) was counted as the leap day (month), but nowadays February 29 is regarded as the leap day in most countries.
Although the calendar year runs from January 1 to December 31, sometimes year numbers were based on a different starting point within the calendar. Confusingly, the term "Anno Domini" is not specific on this point, and actually refers to a family of year numbering systems with different starting points for the years. (See the section below for more on this issue.)
The motivation of the Catholic Church in adjusting the calendar was to celebrate Easter at the time they thought was agreed to at the First Council of Nicaea in 325. Although a canon of the council implies that all churches used the same Easter, they did not. The Church of Alexandria celebrated Easter on the Sunday after the 14th day of the Moon (computed using the Metonic cycle) that falls on or after the vernal equinox, which they placed on March 21,. However, the Church of Rome still regarded March 25, as the equinox and used a different cycle to compute the day of the Moon. By the tenth century all churches (except for some on the eastern border of the Byzantine Empire) had adopted the Alexandrian Easter, which still placed the vernal equinox on March 21,, although Bede had already noted its drift in 725—it had drifted even further by the sixteenth century.
Worse, the reckoned Moon that was used to compute Easter was fixed to the Julian year by a 19 year cycle. However, that approximation built up an error of one day every 310 years, so by the sixteenth century the lunar calendar was out of phase with the real Moon by four days.
for a more consistent and accurate scheduling of the feast of Easter.
The fix was to come in two stages. First, it was necessary to approximate the correct length of a solar year. The value in the major astronomical tables of the day. Although close to the mean tropical year of 365.24219 days, it is even closer to the vernal equinox year of 365.2424 days; this fact made the choice of approximation particularly appropriate as the purpose of creating the calendar was to ensure that the vernal equinox would be near a specific date (March 21). (See Accuracy).
The second stage was to devise a model based on the approximation which would provide an accurate yet simple, rule-based calendar. The formula designed by Aloysius Lilius was ultimately successful. It proposed a 10-day correction to revert the drift since Nicaea, and the imposition of a leap day in only 97 years in 400 rather than in 1 year in 4. To implement the model, it was provided that years divisible by 100 would be leap years only if they were divisible by 400 as well. So, 2000 and 2400 are leap years, but 1700, 1800, 1900, 2100, 2200 and 2300 are non-leap years. This theory was expanded upon by Christopher Clavius in a closely argued, 800 page volume. He would later defend his and Lilius's work against detractors.
The 19-year cycle used for the lunar calendar was also to be corrected by one day every 300 or 400 years (8 times in 2500 years) along with corrections for the years (1700, 1800, 1900, 2100, 2200, 2300, 2500, etc.) that are no longer leap years. In fact, a new method for computing the date of Easter was introduced.
Lilius originally proposed that the 10-day correction should be implemented by deleting the Julian leap day on each of its ten occurrences during a period of 40 years, thereby providing for a gradual return of the equinox to March 21,. However, Clavius's opinion was that the correction should take place in one move and it was this advice which prevailed with Gregory. Accordingly, when the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of ten days. The last day of the Julian calendar was Thursday October 4, 1582 and this was followed by the first day of the Gregorian calendar, Friday October 15, 1582 (the cycle of weekdays was not affected). Nevertheless, the dates "October 5, 1582" to "October 14, 1582" (inclusive) are still valid in virtually all countries because most non-Catholic countries initially rejected the reform and even most Catholic countries did not adopt the new calendar on the date specified by the bull but only with some delay.
Only Spain and her territories, Portugal, the Polish-Lithuanian Commonwealth, and most of Italy implemented the new calendar on Friday, October 15, 1582, following Julian Thursday, October 4, 1582. France adopted the new calendar on Monday, December 20, 1582, following Sunday, December 9, 1582. The Protestant Dutch provinces of Holland and Zeeland also adopted it in December of that year.
Most non-Catholic countries initially objected to adopting a Catholic invention. Although Scotland changed the beginning of the year to January 1, in the year 1600, like England and thereby the rest of the British Empire (including the eastern part of what is now the United States), Scotland did not adopt the Gregorian calendar until 1752; by which time it was necessary to correct by eleven days (Wednesday, September 2, 1752 being followed by Thursday, September 14, 1752) to account for February 29, 1700 (Julian). A few years later, when the son of the Earl of Macclesfield (who had been influential in passing the calendar law) ran for a seat in Parliament in Oxfordshire as a Whig in 1754, dissatisfaction with the calendar reforms was one of a number of issues raised by his Tory opponents. In 1755, William Hogarth made a painting (and an engraved print from the painting) loosely based on these elections, in which the campaign slogan "Give us our Eleven Days" appears (on floor at lower right); this was later misunderstood, giving rise to apocryphal stories of widespread riots at the change-over.
Great Britain legislated special provisions to make sure that monthly or yearly payments would not become due until the dates that they originally would have in the Julian calendar. From 1753 until 1799, the tax year in Great Britain began on April 5,, which was the "old style" new year of March 25,. A 12th skipped Julian leap day in 1800 changed its start to April 6,. It was not changed when a 13th Julian leap day was skipped in 1900, so the tax year in the United Kingdom is still April 6,.
"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which system is used in the British Empire and other countries that did not immediately change. In Britain it is usual to map most dates from the Julian year onto the Gregorian year without converting the day and month. But because the start of the year did not change until the same year that the Gregorian calendar was introduced, OS/NS is particularly relevant for dates which fall between January 1, and March 25,. For example the execution of King Charles I is usually recorded as having taken place on January 30, 1649 (NS), but in contemporary documents it is recorded as having taken place on January 30, 1648.
Denmark, Norway and the Protestant states of Germany adopted the solar portion of the new calendar on Monday, March 1, 1700,, following Sunday, February 18, 1700, due to the influence of Ole Rømer, but did not adopt the lunar portion. Instead, they decided to calculate the date of Easter astronomically using the instant of the vernal equinox and the full moon according to Kepler's Rudolphine Tables of 1627. They finally adopted the lunar portion of the Gregorian calendar in 1776. The remaining provinces of the Dutch Republic also adopted the Gregorian calendar in 1700.
Sweden's relationship with the Gregorian Calendar had a difficult birth. Sweden started to make the change from the OS calendar and towards the NS calendar in 1700, but it was decided to make the (then 11 day) adjustment gradually, by excluding the leap days (February 29,) from each of 11 successive leap years, 1700 to 1740. In the meantime, not only would the Swedish calendar be out of step with both the Julian calendar and the Gregorian calendar for 40 years, but also the difference would not be static but would change every 4 years. This strange system clearly had great potential for endless confusion when working out the dates of Swedish events in this 40 year period. To make matters worse, the system was poorly administered and the leap days that should have been excluded from 1704 and 1708 were not excluded. The Swedish calendar should by now have been 8 days behind the Gregorian, but it was still in fact 10 days behind. King Charles XII wisely recognised that the gradual change to the new system was not working and he abandoned it. However, rather than now proceeding directly to the Gregorian calendar (as in hindsight seems to have been the sensible and obvious thing to do), it was decided to revert to the Julian calendar. This was achieved by introducing the unique date February 30, in the year 1712, adjusting the discrepancy in the calendars from 10 back to 11 days. Sweden finally adopted the Gregorian calendar in 1753, when Wednesday, February 17, was followed by Thursday, March 1,.
In Alaska, the change took place when Friday, October 6, 1867 was followed again by Friday, October 18 after the US purchase of Alaska from Russia, which was still on the Julian calendar. Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because the International Date Line was shifted from east of to west of Alaska along with the change to the Gregorian calendar.
In Russia the Gregorian calendar was accepted after the October Revolution (so named because it took place in October 1917 in the Julian calendar). On January 24, 1918 the Council of People's Commissars decreed that Wednesday, January 31, 1918 was to be followed by Thursday, February 14, 1918.
The last country of Eastern Orthodox Europe to adopt the Gregorian calendar was Greece on Thursday, March 1, 1923, following Wednesday, February 15, 1923. However, these were all civil adoptions—none of the national churches accepted it. Instead, a Revised Julian calendar was proposed in May 1923 which dropped 13 days in 1923 and adopted a different leap year rule that resulted in no difference between the two calendars until 2800. The Orthodox churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Poland, and Bulgaria adopted the Revised Julian calendar, so these New calendarists will celebrate the Nativity along with the Western churches on December 25, in the Gregorian calendar until 2800. The Orthodox churches of Jerusalem, Russia, Serbia, Georgia and the Greek Old Calendarists did not accept the Revised Julian calendar. These Old Calendarists continue to celebrate the Nativity on December 25, in the Julian calendar, which is January 7, in the Gregorian calendar until 2100. All of the other Eastern churches, the Oriental Orthodox churches (Coptic, Ethiopian, Eritrean, Syrian, Armenian) and the Assyrian Church, continue to use their own calendars, which usually result in fixed dates being celebrated in accordance with the Julian calendar. All Eastern churches continue to use the Julian Easter with the sole exception of the Finnish Orthodox Church, which has adopted the Gregorian Easter.
Adoption in East AsiaEdit
The Republic of China (ROC) formally adopted the Gregorian calendar at its founding on January 1, 1912, but China soon descended into a period of warlordism with different warlords using different calendars. With the unification of China under the Kuomintang in October 1928, the Nationalist Government decreed that effective January 1, 1929 the Gregorian calendar would be used henceforth. However, China retained the Chinese traditions of numbering the months and a modified Era System, backdating the first year of the ROC to 1912; this system is still in use in Taiwan where this ROC government retains control. Upon its foundation in 1949, the People's Republic of China continued to use the Gregorian calendar with numbered months, but abolished the ROC Era System and adopted the Western fashion of naming years.
Japan replaced the traditional lunisolar calendar with the Gregorian calendar on January 1, 1873, but, like China, continued to number the months, and used reign names instead of the Common Era: Meiji 1=1868, Taisho 1=1912, Showa 1=1926, Heisei 1=1989, and so on. The "Western calendar" (西暦, seireki) using western year numbers, is also widely accepted by civilians and to a lesser extent by government agencies.
Korea started using the Gregorian calendar on January 1, 1896 due to Japanese influence. The lunisolar Korean calendar used immediately before that day was based on the lunisolar Chinese calendar.
Proleptic Gregorian calendarEdit
The Gregorian calendar can, for certain purposes, be extended backwards to dates preceding its official introduction, producing the proleptic Gregorian calendar. However, this proleptic calendar should be used with great caution.
For ordinary purposes, the dates of events occurring prior to October 15, 1582 are generally shown as they appeared in the Julian calendar, and not converted into their Gregorian equivalents.
However, events occurring in countries where the Gregorian calendar was introduced later than October 4, 1582 are a little more contentious. For example, in Great Britain and its overseas possessions (then including the American colonies), the new calendar was not introduced until September 14, 1752. How, then, would people date events occurring in Britain and her possessions in the 170 years between 1582 and 1752? The answer depends very much on the context, but writers who want to avoid confusion make it absolutely clear which calendar is being used. People have avoided changing historical records in Britain deriving from this period; however, it is often highly desirable to translate particular Old Style dates into their New Style equivalents, such as where the context includes reference to other countries that had already converted to New Style before Britain did. Astronomers avoid this ambiguity by the use of the Julian day number.
If comparisons of dates are done using different calendars, we can encounter logical absurdities such as William and Mary of Orange seeming to arrive in London to accept the English crown, a week or so before they left the Netherlands; and Shakespeare and Cervantes apparently dying on exactly the same date (April 23, 1616), when in fact Cervantes predeceased Shakespeare by 10 days in real time. This coincidence however has allowed UNESCO to make April 23 the World Book and Copyright Day.
For dates before the year 1, unlike the proleptic Gregorian calendar used in the international standard ISO 8601, the traditional proleptic Gregorian calendar (like the Julian calendar) does not have a year 0 and instead uses the ordinal numbers 1, 2, ..., both for years AD and BC and for CE and BCE. Thus the traditional timeline is 2 BC, 1 BC, AD 1, and AD 2. ISO 8601 uses astronomical year numbering which includes a year 0 and negative numbers before it. Thus the ISO 8601 timeline is -0001, 0000, 0001, and 0002.
Confusion with British versus American usageEdit
Dates of events in Britain prior to 1752 are usually now shown in their original Old Style form, whereas dates of events in (then British) America prior to 1752 are usually now shown in the New Style form.
For example, Shakespeare died on April 23, (OS), and it is rare to see this converted to May 3, (NS). But while George Washington was born on February 11, (OS), his birthday is now celebrated on February 22, (NS), even though he himself continued to celebrate his birthday on February 11. However, neither of these practices is universal in either country, so it is sometimes very unclear which calendar is being used, and this can lead to false assumptions, which can lead to dates being inaccurately converted from one calendar to the other. Since the resurgence of interest in the history of the calendar, more information about the real dates (according to various calendars) of events has been forthcoming and many previous errors have been corrected. While these changes are welcome, there is still much scope for confusion; specifically noting the calendar being used can help the reader understand the dates involved.
Months of the yearEdit
- Thirty days hath September,
- April, June, and November.
- All the rest have thirty-one,
- excepting February alone,
- which hath twenty-eight.
- Leap year cometh one year in four,
- in which February hath one day more.
(The hath in the first line of the poem is also given as has or have.)
Alternate endings include:
- excepting February alone,
- which has twenty-eight days or,
- in a leap year, adds one more.
- which has but twenty-eight, in fine,
- till leap year gives it twenty-nine.
- which has eight and a score,
- until leap year gives it one day more.
- which hath twenty-eight days clear,
- and twenty-nine in each leap year.
- in each leap we assign,
- February twenty-nine.
- When short February's done,
- all the rest have thirty-one.
- (except February,)
- February alone don't hold the line,
- for three years it has twenty-eight,
- and the fourth year twenty-nine.
- but February, it is done
- at twenty-eight, but add one more
- whenever the year divides by four.
A shorter, satirical modern alternate ending is:
- but silly old February spoils the fun.
A language-independent alternative used in many countries is to hold up your two fists with the index knuckle of your left hand against the index knuckle of your right hand. Then, starting with January from the little knuckle of your left hand, count knuckle, space, knuckle, space through the months. A knuckle represents a month of 31 days, and a space represents a short month (a 28- or 29-day February or any 30-day month). The junction between the hands is not counted, so the two index knuckles represent July and August. This method also works by starting the sequence on the right hand's little knuckle, and continue toward to the left. You can also use just one hand; after counting the fourth knuckle as July, start again counting the first knuckle as August.
The Origins of English naming used by the Gregorian calendar:
- January: Janus (Roman god of gates, doorways, beginnings and endings)
- February: Februus] (Etruscan god of death) Februarius (mensis) (Latin for "month of purification (rituals)" it is said to be a Sabine word, the last month of ancient pre-450 BC Roman calendar)
- March: Mars (Roman god of war)
- April: Aprilis (mensis) (Latin for "month of Venus," second month of ancient Roman calendar)
- May: Maia Maiestas (Roman goddess)
- June: Juno (Roman goddess, wife of Jupiter)
- July: Julius Caesar] (Roman dictator)
- August: Augustus (first Roman emperor)
- September: septem (Latin for seven, the seventh month in the calendar of Romulus and Remus|Romulus)
- October: octo (Latin for eight, the eighth month in the calendar of Romulus)
- November: novem (Latin for nine, the ninth month in the calendar of Romulus)
- December: decem (Latin for ten, the tenth month in the calendar of Romulus)
The Gregorian calendar improves the approximation made by the Julian calendar by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long, which has an error of about one day per 3300 years with respect to the mean tropical year of 365.24219 days but less than half this error with respect to the vernal equinox year of 365.24237 days. Both are substantially more accurate than the one day in 128 years error of the Julian calendar (average year 365.25 days).
In the 19th century, Sir John Herschel proposed a modification to the Gregorian calendar with 969 leap days per 4000 years, instead of 97 leap days per 400 years, thus reducing the average year to 365.24225 days. However, Herschel's proposal was never accepted.
On timescales of thousands of years, the Gregorian calendar falls behind the seasons drastically because the slowing down of the Earth's rotation makes each day slightly longer over time (see tidal acceleration and leap second) while the year maintains a more uniform duration. The equinox will occur earlier than now by a number of days approximately equal to [years into future/5000]2. This is a problem that the Gregorian calendar shares with any fixed rule-based calendar.
When leap years and common years are taken into account, there are a total of 14 possible Gregorian calendars. When the five different weeks in which Easter can fall are also taken into account, there are a total of 70 possible Gregorian calendars.
The 70 possible Gregorian calendars have a wide range of frequency of occurrence. The commonest Gregorian calendar is a common year starting on Thursday with Easter in week 5 (April 19) which occurs in 2.93% of all years, or once every 34.1 years on average. This year layout last occurred in 1987 and will next occur in 2071. The rarest year layout is a leap year starting on Wednesday with Easter in week 1 (March 22) which occurs in 0.117% of all years, or on average once every 857 years. This year layout has never occurred since 1583, the first Easter after the Gregorian reform in 1582. It will occur for the first time in 2972, assuming the Gregorian calendar is not reformed before then.
An average year is 365.2425 days = 52.1775 weeks = 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds. All these numbers are exact, apart from leap seconds.
A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.
A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.
Since 1971, some years may also contain one or more leap seconds, to account for cumulative irregularities in the Earth's rotation. So far, these have always been positive and have occurred on average once every 18 months.
The day of the year is somewhat inconvenient to compute, not in the least because of the leap day somewhere in the middle; but the calendar has this repeating pattern for the months March through July and August through December: 31, 30, 31, 30, 31 days, totalling 153 days. In fact, any 5 consecutive months not containing February, count 153 days. 153 happens to be the 17th triangular number, and the sum of the first 5 factorials (among other numerical trivia).
The 400-year cycle of the Gregorian calendar has 146,097 days and hence exactly 20,871 weeks. So, for example, the days of the week in Gregorian 1603 were exactly the same as for 2003. This also causes more months to begin on a Sunday (and hence have Friday 13) than any other day of the week (see below for a more detailed explanation of how this happens). 688 out of every 4800 months (or 172/1200) begin on a Sunday, while only 684 out of every 4800 months (171/1200) begin on each of Saturday and Monday, the least common cases.
A smaller cycle is 28 years (1,461 weeks), provided that there is no dropped leap year in between. Days of the week in years may also repeat after 6, 11, 12, 28 or 40 years. Intervals of 6 and 11 are only possible with common years, while intervals of 28 and 40 are only possible with leap years. An interval of 12 years only occurs with common years when there is a dropped leap year in between.
The Doomsday algorithm is a method by which you can discern which of the 14 calendar variations should be used in any given year (after the Gregorian reformation). It is based on the last day in February, referred to as the Doomsday.
The Gregorian serial date, also called Rata Die, is the number of days from January 1, 1 A.D. (counting that day as day 1). For January 31, 2007, the serial date is 732707. It is 678576 more than the Modified Julian date, and 1721425 less than the Julian date 2454132.
In conjunction with the system of months there is a system of weeks. A physical or electronic calendar provides conversion from a given date to the weekday, and shows multiple dates for a given weekday and month. Calculating the day of the week is not very simple, because of the irregularities in the Gregorian system.
The ISO week date connects Gregorian years and weeks, defining a leap week calendar with so-called "ISO years" deviating at the beginning and end up to 3 days from Gregorian years, and with week numbers by year.
Origins of English naming used by the Gregorian Calendar:
- Sunday - sun day (celestial)
- Monday - moon day (celestial)
- Tuesday - Tyr's day (Old Norse god - Tiw in Old English, Teiw in Proto-Germanic)
- Wednesday - Woden's day (Old English god - Norse Odin, German Wotan))
- Thursday - Thor's day (Old Norse god)
- Friday - Frigg's day (Old Norse goddess) (Friday is often erroneously associated with Freyja)
- Saturday - Saturn's day (Roman god)
Distribution of dates by day of the weekEdit
Because there are 97 leap years in every 400 years in the Gregorian Calendar, there are on average 136⁄7 for each starting weekday in each cycle. This already shows that the frequency is not the same for each weekday, which is due to the effects of the "common" centennial years (1700, 1800, 1900, 2100, 2200 etc.).
The absence of an extra day in such years causes the following leap year (1704, 1804, 1904, 2104 etc.) to start on the same day of the week as the leap year twelve years before (1692, 1792, 1892, 2092 etc.). Similarly, the leap year eight years after a "common" centennial year (1708, 1808, 1908, 2108 etc.) starts on the same day of the week as the leap year immediately prior to the "common" centennial year (1696, 1796, 1896, 2096 etc.). Thus, those days of the week on which such leap years begin gain an extra year or two in each cycle. In each cycle there are:
- 13 leap years starting on Monday
- 14 leap years starting on Tuesday
- 14 leap years starting on Wednesday
- 13 leap years starting on Thursday
- 15 leap years starting on Friday
- 13 leap years starting on Saturday
- 15 leap years starting on Sunday
Note that as a cycle, this pattern is symmetric with respect to the low Saturday value.
A leap year starting on Sunday means the next year does not start on Monday, so more leap years starting on Sunday means fewer years starting on Monday, etc. Thus the pattern of number of years starting on each day is inverted and shifted by one weekday: 56, 58, 57, 57, 58, 56, 58 (symmetric with respect to the high Sunday value).
The number of common years starting on each day is found by subtraction: 43, 44, 43, 44, 43, 43, 43.
The frequency of a particular date being on a particular weekday can easily be derived from the above (for dates in March and later, relate them to the next New Year).
See also the cycle of Doomsdays.
January 1 of any year whose number is a multiple of 400 is a Saturday. From this you can work out the day of the week of any date.
The Roman calendar was modified by Julius Caesar when he occupied the office of Pontifex Maximus and the Julian calendar was subsequently modified by Gregory XIII, who, as pope, also held the title Pontifex Maximus.
Non-leap years always begin and end on the same day of the week, since 364 (365 - 1) is a multiple of 7, the number of days in a week. For example, 2003 began on a Wednesday and ended on a Wednesday. Leap years end on the next day of the week from which they begin. For example, 2004 began on a Thursday and ended on a Friday.
Not counting leap years, any calendar date will move to the next day of the week the following year. For example, if your birthday fell on a Tuesday in 2002, it fell on a Wednesday in 2003. Leap years make things a little more complicated. 2004 was a leap year, so calendar days of March 1 or later in the year, moved two days of the week from 2003. However, calendar days occurring before March 1 do not make the extra day of the week jump until the year following a leap year. So, if your birthday is June 15, then it must have fallen on a Sunday in 2003 and a Tuesday in 2004. If, however, your birthday is February 15, then it must have fallen on a Saturday in 2003, a Sunday in 2004 and a Tuesday in 2005.
In any year (even a leap year), July always begins on the same day of the week that April does. Therefore, the only difference between a July calendar page and an April calendar page in the same year is the extra day July has. The same relationship exists between September and December as well as between March and November. Add an extra day to the September page and you've got December. Take a day away from the March page and you've got November. In non-leap years only, there are additional matches: October duplicates January, and March and November duplicate February in their first 28 days. In leap years only, there is a different set of additional matches: July is a duplicate of January while February is duplicated in the first 29 days of August.
Saint Teresa of Ávila died on the night from October 4, to October 15, 1582, that is, exactly when Spain and the Catholic world switched to the Gregorian calendar.
- Gregorian reform of the calendar: Proceedings of the Vatican conference to commemorate its 400th anniversary, 1582-1992, ed. G. V. Coyne, M. A. Hoskin, and O. Pedersen (Vatican City: Pontifical Academy of Sciences, Specolo Vaticano, 1983).
- The Oxford Companion to the Year. Bonnie Blackburn & Leofranc Holford-Strevens. Oxford University Press 1999. ISBN 0-19-214231-3. Pages 98-99.
- Calendar: Humanity's Epic Struggle To Determine A True And Accurate Year, David Ewing Duncan, Harper Perennial, 1999, ISBN 0-380-79324-5.
- Online Etymology Dictionary retrieved August 23, 2006
- Calculation of Julian day
- Calendar reform
- Greek Old Calendarists
- List of calendars
- Old Style and New Style dates
- Year zero
- Inter Gravissimas, Gregory XIII's bull introducing the new calendar (Latin and French)
- Inter Gravissimas (Latin and French plus English)
- British Calendar Act 1751
- Frequently Asked Questions about Calendars
- The Perpetual Calendar Gregorian Calendar adoption dates for many countries.
- Synoptical Julian - Gregorian calendar Compare Old and New Style dates 1582 - 2100.
- Gregorian Calendar Printer
- Gregorian Calendar in norwegian, with some norwegian information
- Use of calendars in Scotland
- The Julian and Gregorian Calendars