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5-Week Leap Month Calendar

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The 5-Week Leap Month Calendar has 10 months of exactly 5 weeks, i.e. 35 days, in common years and an additional 11th month of equal length in leap years. Since the year is thus either way too short at 350 days or way too long at 385 days, it’s not really an appropriate solar calendar and the leap month is inserted quite often (43.55%, i.e. almost every other year); the months are also too long to make it a good lunar calendar.

Unfortunately, none of the commonly used or proposed leap cycles contains an integer multiple of 5 weeks and those that do and are shorter than a millennium yield a worse average year than the Gregorian leap cycle.

Years Leaps Months Weeks Average days
62 27 647 3235 365.2419355
969 422 10112 50560 365,2425181
907 395 9465 47325 365.2425579
845 368 8818 44090 365.2426036
783 341 8171 40855 365.2426564
721 314 7524 37620 365.2427184
659 278 6877 34385 365.2427921
597 260 6230 31150 365.2428811
535 252 5583 27915 365.2429907
473 206 4936 24680 365.243129
411 179 4289 21445 365.243309
349 152 3642 18210 365.243553
287 125 2995 14975 365.2439024

Like its sibling, the 4-Week Leap Month Calendar, this calendar follows ISO 8601 conventions as far as possible, i.e. all weeks, months and years start on a Monday. The date notation is YYYY-MM-W-D, where MM ranges from 01 to 11, W from 1 to 5 and D from 1 to 7.

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