Intercalate

verb

1. To insert an extra leap day into a calendar in order to maintain synchrony with natural phenomena.
2. To insert an extra month into a calendar for the same purpose. The Hebrew calendar has such a month.
3. To insert a substance between two or more molecules, bases, cells, or tissues.
4. To insert anything somewhere (especially between other things), such as an affix into a word. (Compare interpolate.)