According to the ruling of the First Ecumenical Synod in 325, Easter Sunday should fall on the Sunday which follows the first full moon after the vernal equinox. If the full moon happens to fall on a Sunday, Easter is observed the following Sunday. The day taken to be the invariable date of the vernal equinox is March 21. | According to the ruling of the First Ecumenical Synod in 325, Easter Sunday should fall on the Sunday which follows the first full moon after the vernal equinox. If the full moon happens to fall on a Sunday, Easter is observed the following Sunday. The day taken to be the invariable date of the vernal equinox is March 21. |