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For other uses, see Intercalation (disambiguation). Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months. The solar year does not have a whole number of days (it's about 365.24 days), but a calendar year must have a whole number of days. The only way to reconcile the two is to vary the number of days in the calendar year. In solar calendars, this is often done by adding to a common year of 365 days, an extra day ("leap day" or "intercalary day") about every four years: this makes a leap year of 366 days. The Decree of Canopus, which was issued by the pharaoh Ptolemy III, Euergetes of Ancient Egypt in 239 BC, decreed a solar leap day system. In the Julian Calendar, as well as in the Gregorian Calendar, which improved upon it, intercalation is done by adding an extra day to February in each leap year. In the Julian Calendar this was done every four years. In the Gregorian, years divisible by 100 but not 400, were exempted in order to improve accuracy. Thus, 2000 was a leap year; 1700, 1800, and 1900 were not. The solar year does not have a whole number of lunar months (it is about 12.37 lunations), so a lunisolar calendar must have a variable number of months in a year. This is usually 12, but a 13th (an "intercalary" or "embolismic" month) is inserted every second or third year. ISO 8601 includes a specification for a 52-week year. Any year that has 53 Thursdays has 53 weeks; this extra week may be regarded as intercalary. The determination of whether a year has intercalation may be calculated (Julian, Gregorian and Hebrew calendars), or determined by observation (Iranian calendar). The International Earth Rotation and Reference Systems Service can insert or remove leap seconds from the calendar up to twice a year. These are sometimes described as intercalary,[1] although the word intercalation strictly applies only to periods of 24 hours or longer. [edit] See also
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