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A leap week calendar is a reformed calendar system in which each year consists of a whole number of weeks. Leap years have one week (or some other whole number of weeks) added, so each year begins on the same day of the week.
The ISO week is an example of such a calendar. It is a variation of the Gregorian calendar that is used (mainly) in government and business for fiscal years, as well as in timekeeping. In this system a year (ISO year) has 52 or 53 full weeks (364 or 371 days).
Leap week calendars vary on whether the concept of month is preserved and whether the month (if preserved) has a whole number of weeks.
Most leap week calendars take advantage of the fact that 400 Gregorian calendar years have exactly 20,871 weeks, so with non-leap years of 52 weeks, this means there are 71 leap weeks every 400 years. These include the Pax Calendar and CCC&T, as well as the ISO week dates.
One proposed calendar (The Bonavian Civil Calendar) has 159 leap weeks in a 896-year cycle, which has a mean year of 365.2421875 days. Josef Suran's World Calendar with Leap Weeks has a 62 year cycle with 11 leap weeks, which has a mean year of 365.241935 days. The Symmetry454 Calendar and others have a 293-year cycle with 52 leap weeks equivalent to 294 years of 52 weeks.
Note that the new years of the calendars shown need not be synchronised.
Leap Year Cycles
|Years||Days||Weeks||Leap Weeks||Mean Year||Comments|
|62||22645||3235||11||365.241935...||World Calendar with Leap Weeks|
|400||146097||20871||71||365.2425||Gregorian 400-year cycle|
|896||327257||46751||159||365.2421875||Bonavian Civil Calendar|