Intercalation

n.	1.
	1.	(Chron.) The insertion of a day, or other portion of time, in a calendar.
	2.	(Geol.) The insertion or introduction of anything among others, as the insertion of a phrase, line, or verse in a metrical composition; Intercalations of fresh-water species in some localities. - Mantell.

Noun

1.

intercalation - an insertion into a calendar Synonyms: embolism

calendar, embolism, interval, time interval