"to insert a day into the calendar," 1610s, from Latin intercalatus, past participle of intercalare "to proclaim the insertion of an intercalary day," from inter "between" (see inter-) + calare "to call" (an intercalary day; see calendar). Sometimes used in a general sense, "to insert between others" (1824). Related: Intercalated; intercalating.
A necessary process in the Roman calendar to balance the solar and lunar aspects of it. Intercalation was done after Feb. 23 or 24 (the terminalia), every two or four years. Twenty-seven days were intercalated, making a full intercalary month (which included the last four or five days of Februarius), known as mensis intercalaris (and also known, according to Plutarch, as Mercedonius). No one now knows why the intercalation was done in the middle of February rather than after its end, unless it was because the important festivals at the end of that month (Regifugium and Equirra) were closely associated with holidays in early March. After Caesar's reform (46 B.C.E.) the only intercalary day is Feb. 29 every four years.