intercalatiemaand

Intercalatiemaand, or intercalary month, is a calendar month inserted into a year to keep the calendar aligned with the solar year or agricultural cycle. Because lunar months are about 29.5 days, twelve lunar months total roughly 354 days, about 11 days shorter than the solar year. Without intercalation, seasons and religious observances would drift over time.

In practice, an intercalary month is added in certain years, yielding 13 months in those years. The most well-known example is the Hebrew calendar, which uses a 19-year Metonic cycle that includes seven leap years in which Adar I and Adar II are both counted, producing 13 months. The extra month ensures that Passover remains in spring. Many traditional lunisolar calendars, including various East Asian systems, insert a leap month when needed, determined by astronomical rules rather than a fixed pattern.

Other historical and cultural systems also employed intercalary months. The ancient Roman calendar occasionally inserted an extra month (such as Mercedonius) to realign the calendar with the seasons, a practice later replaced by Julius Caesar’s reform into a solar-based calendar. In contrast, some calendars resolve drift by adding days rather than months; for example, the modern Gregorian calendar uses leap years with an extra day in February.

Some purely lunar calendars, like the Islamic calendar, do not use intercalary months. They maintain a 12-month year of 354 or 355 days, with drift managed by occasional addition of a leap day rather than a full month.