Months have either 30 or 31 days. Patterns can be simple, i.e. 30:31 and 31:30, quarter-based, i.e. 31:30:30, 30:31:30 and 30:30:31, more complicated ,or totally irregular as in the Gregorian calendar.
On the one hand, six times 61 makes 366 days, thus either a 31st or a 30th day must be left out in leap years. On the other hand, four time 91 makes 364 days, thus either a 31st or a 32nd day must be added in common years, and another 31st, 32nd or 33rd in leap years, or a leap week has to be introduced which may make one month 37 or 38 days long in leap years.
This category has the following 7 subcategories, out of 7 total.
Pages in category "30-31-day month calendars"
The following 4 pages are in this category, out of 4 total.