intercalate

Concise Oxford English Dictionary © 2008 Oxford University Press:
intercalate /ɪnˈtəːkəleɪt/

▶verb

• 1 insert (an intercalary period) in a calendar.

• 2 insert between layers in a crystal lattice or other structure.

– derivatives
intercalation noun.

– origin C17: from L. intercalat-, intercalare ‘proclaim as inserted in the calendar’.