1550s, "a number attached to a year to show the number of days into the calendar moon on which the solar year begins;" 1580s, "number of days by which the solar year exceeds a lunar one of 12 moons;" from French épacte (12c.), from Late Latin epacta "an intercalary day," from Greek epakte (plural epaktai, in epaktai hemerai "intercalary days"), from fem. of epaktos "brought in, imported, alien," verbal adjective of epagein "to add, bring forward," also "intercalate," from epi "on" (see epi-) + agein "to bring, to lead" (see act (v.)). Related: Epactal.