intercalation -
A period inserted into a calendar as in a leap year.
intercalation -
The reversible insertion of a molecule between two others.
intercalation -
A layer introduced into a pre-existing sequence.
Wikipedia
Intercalation is 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.