Origins of Easter
Easter was variously celebrated on different days of the week, including Friday, Saturday, and Sunday. In that year, the Council of Nicaea was convened by emperor Constantine. It issued the Easter Rule which states that Easter shall be celebrated on the first Sunday that occurs after the first full moon on or after the vernal equinox. The “full moon” in the rule is the ecclesiastical full moon, which is defined as the 14th day of a tabular lunation, where day 1 corresponds to the ecclesiastical New Moon. It does not always occur on the same date as the astronomical full moon.