The Rulings of the Council of Nicaea on the Date of Easter by Rogelio G. Dela Cruz, Ph.D (April, 2006)

Easter is a festival commemorating the resurrection of Jesus Christ, and the principal feast of the Christian year. It is celebrated on a Sunday on varying dates between March 22 and April 25 and is therefore called a movable feast.

Connected with the observance of Easter are the 40-day penitential season of Lent, beginning on Ash Wednesday and concluding at midnight on Holy Saturday, the day before Easter Sunday; Holy Week, commencing on Palm Sunday, including Good Friday, the day of the crucifixion, and terminating with Holy Saturday; and the Octave of Easter, extending from Easter Sunday through the following Sunday. During the Octave of Easter in early Christian times, the newly baptized wore white garments, white being the liturgical color of Easter and signifying light, purity, and joy.

Constantine the Great, Roman emperor, convoked the Council of Nicaea in 325 A.D.. The council unanimously ruled that the Easter festival should be celebrated throughout the Christian world on the first Sunday after the full moon following the vernal equinox; and that if the full moon should occur on a Sunday and thereby coincide with the Passover festival, Easter should be commemorated on the Sunday following. Coincidence of the feasts of Easter and Passover was thus avoided.

The Council of Nicaea also decided that the calendar date of Easter was to be calculated at Alexandria, then the principal astronomical center of the world. The accurate determination of the date, however, proved an impossible task in view of the limited knowledge of the 4th-century world. The principal astronomical problem involved was the discrepancy, called the epact, between the solar year and the lunar year. The chief calendric problem was a gradually increasing discrepancy between the true astronomical year and the Julian calendar then in use.

Ways of fixing the date of the feast tried by the church proved unsatisfactory, and Easter was celebrated on different dates in different parts of the world. In 387 A.D., for example, the dates of Easter in France and Egypt were 35 days apart. About 465 A.D., the church adopted a system of calculation proposed by the astronomer Victorinus (flourished 5th century), who had been commissioned by Pope Hilarius to reform the calendar and fix the date of Easter. Elements of his method are still in use, although the Scythian monk Dionysius Exiguus made significant adjustments to the Easter cycle in the 6th century. Refusal of the British and Celtic Christian churches to adopt the proposed changes led to a bitter dispute between them and Rome in the 7th century.