Intercalation's 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 doesn't have a whole number of days (it's about 365.24 days), but a calendar year must've a whole number of days. The only way to reconcile the two's to vary the number of days in the calendar year. In solar calendars, this's often done by adding to a common year of 365 d… (More on Intercalation)