epact

From French épacte, from Latin epactae, from Ancient Greek ἐπακταί (epaktaí, “intercalary days”), feminine plural of ἐπακτός (epaktós, “brought on or in, added”).

epact (plural epacts)

1. the time (number of days) by which a solar year exceeds twelve lunar months; it is used in the calculation of the date of Easter

• Petac