Intercalation is the insertion of an extra day, week or month into some calendar years to make the calendar follow the seasons. In the Gregorian calendar in common use, the calendar year begins on January 1, and ends on December 31. ... A calendar is a system for naming periods of time, typically days. ... This article is about divisions of a year. ...

The solar year does not have whole number of 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. A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... In the Gregorian calendar in common use, the calendar year begins on January 1, and ends on December 31. ...

In many calendars, this is done by adding to a common year of 365 days, an extra day (leap day or intercalary day): this makes a leap year of 366 days. In the Gregorian calendar, the intercalary day is February 29. A common year is a calendar year of exactly 365 days and so is not a leap year. ... A leap year (or intercalary year) is a year containing an extra day or month in order to keep the calendar year in sync with an astronomical or seasonal year. ... The Gregorian calendar is the calendar widely used in the Western world. ... February 29 is the 60th day of a leap year in the Gregorian Calendar, with 306 days remaining. ...

The solar year does not have a whole number of lunar months either, so a lunisolar calendar must have a variable number of months in a year. This is usually 12 months, but sometimes a 13th month (an intercalary or embolismic month) is added to the year. A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... In lunar calendars, a lunar month is the time between two successive similar syzygies (new moons or full moons). ... A lunisolar calendar is a calendar whose date indicates both the moon phase and the season. ... In Egyptian mythology, Month is an alternate spelling for Menthu. ...

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. ISO 8601 is an international standard for date and time representations. ... Thursday is the day of the week between Wednesday and Friday. ...

The determination of whether a year has intercalation may be calculated (Julian, Gregorian and Hebrew calendars), or determined by observation (Iranian calendar). The Julian calendar was introduced by Julius Caesar in 46 BC, taking force in 45 BC or 709 ab urbe condita. ... The Gregorian calendar is the calendar widely used in the Western world. ... This figure, in a detail of a medieval Hebrew calendar, reminded Jews of the palm branches (Lulav) and the citron (Etrog) to be brought to the synagogue at the end of sukkot, closing the solemn convocations of the calendar in autumn. ... This article is in need of attention. ...

See also

• calendar
• Julian calendar
• Gregorian calendar
• Iranian calendar
• Hebrew calendar
• Hindu calendar
• Chinese calendar
• leap second