The date of the celebration of Easter was defined by the First Ecumenical Council of Nicaea in the year 325 A.D. In this First Ecumenical Council the entire Christian Church of the East and West was represented by 318 Bishops; therefore, no authority or Church could oppose or change this unanimous decision, unless through another Ecumenical Council.

This Holy Council of Nicaea, in the year 325 A.D. decreed the regulations for the calculation of the date of Easter for the whole of Christianity, so that all Christians might celebrate Easter on the same day every year.

These regulations of the First Ecumenical Council are based on the Seventh Apostolic Canon, which reads as follows: "If a Bishop or Priest, or Deacon celebrates the Holy Day of Easter before the vernal equinox, or with the Jews, let him be deposed."

The regulations of the First Ecumenical Council concerning the calculation of the date of Easter were handed down to us by the Council of Antioch in 341 A.D., which had received the decision concerning Easter from the First Ecumenical Council. This is also corroborated by the testimonies of Athanasius the Great and St. Epiphanius of Cyprus.

These regulations of the First Ecumenical Council are as follows:
1. "That Easter must always be celebrated on a Sunday."
2. "That Easter must never be celebrated on the same day as the Jewish Passover."
3. "That Easter should never be celebrated on or before the vernal equinox of any year."

It should also be noted here, that Cyril the Patriarch of Alexandria, in his Paschal Circular, stated:

"The Ecumenical Council unanimously voted that the Church of Alexandria, because of its noted astronomers, would announce to the Church of Rome every year the date of Easter, and Rome in turn would announce it to the other Churches."

This did not mean that the Church of Rome would determine the date of Easter, but that she would announce it to the Churches of the West, after the date was determined by the Church of Alexandria.
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