Calendar year

A calendar year begins on the New Year's Day of the given calendar system and ends on New Year's Eve, and thus consists of a whole number of days. The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days. As this is not an integer, solar calendars have strategies to adjust for the 0.24219 remainder. Neither is it a whole number of lunar months (being about 10 or 11 days more than 12 lunations) so lunisolar calendars also must include adjustments to resynchronise with the seasons. Pure lunar calendars ignore the solar cycle and instead have an invariant year of twelve lunar months that drifts through the seasons. The Gregorian calendar year, which is in use as civil calendar in most of the world, begins on January 1 and ends on December 31. It has a length of 365 days in an ordinary year but, in order to reconcile the calendar year with the astronomical cycle, it has 366 days in a leap year. With 97 leap years every 400 years, the Gregorian calendar year has an average length of 365.2425 days. The corresponding week year has the same mean year, but just 364 days (i.e. 52 full weeks) in most years and 371 days (53 weeks) in 71 out of 400. Other formula-based calendars can have lengths which are further out of step with the solar cycle: The Julian calendar (a solar calendar) has an average length of 365.25 days because it is slightly more 366-day years than the Gregorian calendar. The Hebrew calendar (a lunisolar calendar) has an average length of 365.2468 days, but its calendar years are either 12 months for a total of 354 to 355 days or 13 months (384 days) long. The lunar Hijri calendar ("Islamic calendar") is...