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.
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.
Leap cycle[edit | edit source]
There is one short cycle that is accurate. It is 62 years of 27 leaps and this has a mean year of about 365.2419355 days. There are no cycles more accurate than this and the Gregorian calendar that are less than 1000 years long.