The epact (from Greek: epaktai hèmerai = added days)'s a quantification of the difference between the solar and lunar calendars. It was defined by the second canon of the Gregorian calendar reform as "the number of days by which the common solar year of 365 days surpasses the common lunar year of 354 days". Lunar calendar Epacts are used to find the date in the lunar calendar from the date in the common solar calendar. Solar and lunar years A (solar) calendar year has 365 days (366 days in… (More on Epact)