Intercalation is the insertion of a day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require a combination of both adjustments. 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. In solar calendars, this is often 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.