what does epact mean?

In the context of calendars, an epact is a measure of the age of the lunar month at the time of the vernal equinox. It is the difference, in days, between a lunar calendar and a solar calendar. The term is most commonly used in relation to the Christian liturgical calendar, specifically the calculation of the date of Easter. The epact value is used to determine the date of the full moon that falls on or after the vernal equinox, which is used to calculate the date of Easter.