intercalation

In chronology, an official insertion of additional time, as a day or a month, in the regular reckoning of the calendar, to make the year of the right length. See intercalary, 1.

Hence The insertion of anything between other things; irregular interposition or interjection, as, in geology, the intrusion of layers or beds between the regular rocks of a series.

The insertion of a day, or other portion of time, in a calendar.

The insertion or introduction of anything among others, as the insertion of a phrase, line, or verse in a metrical composition

a period inserted into a calendar as in a leap year.

The reversible insertion of a molecule between two others

A layer introduced into a pre-existing sequence

an insertion into a calendar