Linked from
The 26 pages that link to Intercalation, each with the reason it gives.
Lunar calendarRelated: Lunisolar calendars use it to bring lunar months back toward solar seasons.
Roman CalendarRelated: Roman officials inserted an extra month to correct seasonal drift.
CalendarRelated: It corrects the mismatch between whole calendar units and astronomical periods.
MonthRelated: Calendars use intercalation when ordinary month patterns drift from seasonal time.
Persian calendarRelated: Leap days compensate for the solar year’s fractional day.