The 1,700-Year Quest for a Common Easter

National Catholic Register, 9 April 2025

The Council of Nicaea gave Christianity more than a creed — it gave the Church a way to calculate Easter. But unity around the great feast of the Resurrection remains incomplete.

This year’s 1,700th anniversary of the Council of Nicaea is suitably marked by both the Catholic and Orthodox Easter falling on the same date, which is not the norm. It was at Nicaea that the date of Easter was set; it was one of the most important items on the conciliar agenda.

The anniversary and the joint celebration this year have fostered hopes that a permanent joint celebration of Easter might be possible. That remains unlikely, however, given that relations within the Orthodox world do not permit such a momentous decision to be made. 

Nicaea and Easter

While Nicaea is remembered for the doctrinal defense of the divinity of Christ against the heresy of Arianism — which taught that Christ was not fully divine — the date of Easter was a pressing issue. 

By 325, Constantine had legalized Christianity and strongly supported it as a unifying force in the Roman Empire. One reason for convening the council was to promote imperial unity by resolving theological disputes. Divisions within the increasingly influential religion could destabilize the empire and potentially threaten the emperor’s authority. 

The same was true for religious celebrations, which touched the common life of the people more directly than theological questions. If the entirety of the now-Christian empire could celebrate its most important feast at the same time, it would underscore the unity of far-flung regions under a common rule.

There were already different traditions. Some Christians celebrated Easter at the time of the Jewish Passover. Others observed it on Sunday near Passover. And how to calculate Passover according to the Roman calendar, not the Jewish calendar?

At the time, the Roman Empire followed a calendar established by Julius Caesar in 46 B.C. — the “Julian” calendar. It was a solar-based calendar that remains familiar today: a year of 365 days organized into 12 months of 30 or 31 days, save for February, which had 28 days. Every fourth year was a leap year, with an extra day added to February, as the actual orbit of the Earth around the sun took about 365¼ days.

The challenge of calculating Easter was how to fit the Hebrew and Julian calendars together. The Hebrew calendar assigns great weight to the lunar cycle. Passover was celebrated on the 14th day of the month of Nisan. 

The beginning of the Hebrew months depends upon the phase of the moon. Thus, for Easter to be celebrated in harmony with the recurrence of Passover — as during the original Holy Week — some formula had to be found for incorporating the lunar cycle into the Julian calendar. It was sufficiently complicated that different authorities in different regions disagreed, meaning Easter was celebrated at different times.

Nicaea found a solution. Passover was a springtime feast (in the Northern Hemisphere). The following formula was established and remains to the present day: Easter is the first Sunday after the first full moon (the lunar dimension) after the vernal equinox (the solar dimension). For calculation purposes, the Church fixed the date of the vernal equinox as March 21, even though the astronomical equinox can vary slightly. 

Thus, Easter can fall as early as March 22, if the full moon occurs on a Saturday equinox (March 21). In that case, Easter would be the next day, Sunday, March 22. 

Easter can be as late as April 25, if the full moon after the equinox is not until April 18, which, if a Sunday, means Easter comes on the next Sunday, April 25. 

The Nicaea solution took time to spread, but, eventually, the desired unity was achieved. The question of how to calculate Easter on the calendar was solved. 

Calendar Drift

A problem arose — very gradually. The formula was fine, but the calendar wasn’t. The Julian calculations fixed the year at 365¼ days, meaning an extra “leap” day every four years should keep things in order. But 365¼ days was about 11 minutes too long. Not significant at first, but over the centuries, it added up. 

By the time the Council of Trent met in 1545, the calendar was about 10 days out from where it had started, meaning that the astronomical equinox was falling on March 11 instead of March 21. Since March 21 had been fixed by the Church as the ecclesiastical equinox — the date used to determine Easter — this drift posed a serious problem. The council decided that the calendar should be corrected. This was finally accomplished by Pope Gregory XIII in 1582, nearly 40 years later, when he eliminated 10 dates from the calendar to restore the equinox to March 21. 

The changes came into effect in October. In 1582, people went to bed on Thursday, Oct. 4, and woke up on Friday, Oct. 15. A bit of Catholic trivia: St. Teresa of Ávila died on Oct. 4, 1582. That was already the feast day of St. Francis of Assisi, so she was assigned as a feast the day after she died — not Oct. 5, but Oct. 15.

A further refinement concerned leap years to deal with the 11-minute variance, which was necessary to prevent further drift. The extra day was eliminated in century years if they were not divisible by 400. For example, 1800 and 1900 were not leap years, but 2000 was. That fixed the problem.

Divisions Over Authority

The Council of Trent assumed responsibility for reforming the calendar based on the authority of Nicaea’s work in 325. But by the 16th century, there was no unified empire, no commonly recognized emperor, and a divided Christendom. In 1054, there had been the split between Catholic and Orthodox, and in 1517 the split between Catholic and Protestant. 

While Catholic countries accepted Pope Gregory’s reform — the “Gregorian” calendar — non-Catholic countries did not. Thus, moving around Europe meant not only a time change, but a date change of 10 days. A trip across the border from a Catholic country to a Protestant or Orthodox one meant moving 10 days on the calendar!

That was practically untenable, and despite divisions in Europe, the pope was the only one who could practically assert supranational authority. Eventually, Protestant countries accepted the Gregorian calendar, though it took a long time — Great Britain adopted it only in 1752, nearly two centuries later.

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