Intercalation

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 the vary the number of days in the calendar year.

In many calendars, this is done by having common years of 365 days and leap years of 366 days. This variation is intercalation. The extra day (leap day) may be called an intercalary day.

The solar year does not have a whole number of lunar months, 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 month) is added to the year.

See calendar, Gregorian calendar, Hebrew calendar and Chinese calendar.