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The answer is very complicated as you would expect of religion."By 1583 A.D. the table for determining the Ecclesiastical Full Moon dates was permanently established and has been used ever since to determine the date of Easter. Thus, according to the Ecclesiastical tables, the Paschal Full Moon is the first Ecclesiastical Full Moon date after March 20 (which happened to be the vernal equinox date in 325 A.D.). Thus, in Western Christianity, Easter is always celebrated on the Sunday immediately following the Paschal Full Moon."