The major Christian holy day. It marks Jesus Christ’s Resurrection from the dead three days after his Crucifixion. Western Christian churches and Orthodox Christian churches usually celebrate Easter on different dates, sometimes as much as five weeks apart. Both observe Easter on the first Sunday after the first full moon after the March equinox. However, the Western church uses the Gregorian calendar and the Orthodox church and many Eastern Catholic churches use the Julian calendar. They also use different definitions of a full moon and an equinox. The two Easters are observed on the same day about a quarter of the time. Orthodox Christians refer to Easter as Pascha, derived from the Hebrew word for Passover.
REFERENCE: Religion Stylebook