Why do we celebrate Easter based on a formula of the Roman Catholic Church? Sometimes Easter comes early and sometimes late according to the Gregorian Calendar (named after Pope Gregory) that we use. This is because of a formula created by the Catholic Council of Nicaea in 325 AD which is based on the first full moon of spring. Passover this year is on April 19 so historically the Resurrection would be April 20 however based on this formula Easter is on April 24. Why not celebrate it the day after Passover? Also why do we use the name “Easter” which is of pagan origin?