Setting the date

A number of issues in theology, ecclesiology, and practice separate the Catholic Church from the churches of the Orthodox Communion, a split that dates definitively to the mutual excommunications of the East-West Schism in AD 1054. One relatively small but notable area of disagreement is the date on which Easter Sunday is celebrated.
 
Due to the use of different calendars and different means of calculating the date to commemorate Christ’s Resurrection, Catholic Easter and Orthodox Easter fall on the same Sunday only occasionally. This year, Catholics will mark the Resurrection on April 9, while the Orthodox will celebrate the feast one week later on April 16.

If new discussions between top representatives of the Catholic Church and the Orthodox Communion result in mutual agreement, however, then the two may well celebrate Easter on a common date each year beginning as early as 2025 — a year when the two Easters will coincide anyway, on April 20.

Why is there not already consensus on the Easter date among the churches, and what significance would such agreement bear for eventual Catholic-Orthodox unity?

Turning the calendar

A fundamental issue is that most Orthodox churches still use the Julian calendar, which was introduced by Julius Caesar in 45 B.C., whereas the Catholic Church uses the Gregorian calendar introduced by Pope Gregory XIII in 1582.
 
The Julian calendar had years of 365 days, with every fourth year adding a 366th day. But the actual time it takes for the earth to make one revolution around the sun is over 11 minutes shorter than 365¼ days, so over time the Julian calendar fell into error.
 
Because Easter was not uniformly observed in early Christianity, the Council of Nicaea in AD 325 decreed that Easter should fall on the first Sunday following the first full moon after the vernal equinox. The vernal equinox is the date the sun crosses the celestial equator, the official beginning of spring, which at that time fell on March 21, already having slowly meandered from the Julian calendar’s original equinox date of March 25. Hence, after Nicaea, Easter could fall between March 22 and April 25. But the actual vernal equinox date would keep drifting, and by the 16th century it had shifted to March 11, and Easter was not being observed on the prescribed date.

Church leaders were aware of this inaccuracy and found it scandalous. Seeking to correct the problem under the advice of scientists, Pope Gregory XIII issued a papal bull in 1582 that removed 10 days from the calendar. He kept the system of quadrennial "leap years," but with certain exceptions to compensate for the old Julian drift. When the new calendar was first promulgated, people went to bed one night and essentially woke up 11 calendar "days" later.
 
Uneven reception

Catholic countries promptly adopted the Gregorian calendar, but Protestant countries rejected it until the 1700s. (The American colonies received it in 1752). Russia remained on the Julian calendar until 1917, and Greece did not accept the change until 1923. Today, only five nations — Afghanistan, Eritrea, Ethiopia, Iran, and Nepal — follow something other than the Gregorian calendar for civic use.

Although many Orthodox churches came to accept the Gregorian calendar for fixed religious feasts, all except the Orthodox Church of Finland continue to use the Julian calendar for movable feasts such as Easter.
 
Today, the Julian calendar lags behind the Gregorian calendar by about 13 days. What’s more, the Orthodox churches calculate the Easter date differently from the Catholic Church, observing it after the Jewish celebration of Passover. Christ’s death and resurrection took place after Passover.

Orthodox Easter now falls anywhere between April 4 and May 8, and Catholic Easter still ranges from March 22 to April 25. Some years they coincide, but not in 2023.

In recent dialogue with Pope Francis, however, Ecumenical Patriarch Bartholomew I of Constantinople, successor of St. Andrew the Apostle and "first among equals" among the bishops of the Eastern Orthodox Church, indicated his support for finding a common Easter date, beginning with the natural common date in 2025.
 
But fracturing among Orthodox Christians limits the possibilities for success. In 2018, for example, the Russian Orthodox Church, the largest Orthodox church at nearly 100 million members, announced that it had severed ties with the Ecumenical Patriarchate of Constantinople after Bartholomew confirmed he would recognize the independence of the Ukrainian Orthodox Church. Any agreement between the Catholic Church and the Ecumenical Patriarch would involve the dozens of national Orthodox churches in communion with Constantinople but not those outside of that communion.

Pope Francis has expressed his belief that Catholics and Orthodox will one day attain full communion. While a common date for Easter would have some ecumenical significance, it would mark a relatively minor movement of the needle toward unity. Orthodox and Catholics both accept the first seven ecumenical councils of the Church as well as the seven sacraments and apostolic succession, but differ in key areas as well, most notably papal primacy.
 
Not as simple as it sounds

Father Hugh Barbour, O.Praem., of St. Michael’s Abbey in southern California, an apologist and chaplain at Catholic Answers, noted that Russia’s 2022 invasion of Ukraine has created further divisions between leaders of Orthodox churches and the Ukrainian government, which wants to impose on the Ukrainian Orthodox Church a change to the Gregorian calendar for fixed feasts, thereby "exacerbating the situation horribly and setting back any hope of reunion within Orthodoxy."

Regardless of the political situation, worldwide reunion is desirable but will more likely be attained "bit by bit on a local level," Fr. Barbour said.
 
"While Orthodox churches have the same faith and worship, showing marvelous coherence in liturgy and the doctrine of the faith, their jurisdictions are local and national rather than universal," he explained. "This decentralized power structure makes them hard to deal with ecumenically. The Catholic Church, in contrast, is a papal monarchy able to act from a single center."

He added that while the date of Easter has a symbolic importance, it is not the key issue that papal primacy is.
 
"The Roman church historically never has had any difficulty with Eastern Catholics using the older Julian calendar if they desire," said Fr. Barbour. In fact, there are some benefits. The Orthodox also keep the Julian observance of Christmas on January 7, which along with the different Easter date help separate them from the secular, commercialized observance of such holidays, "making them more tranquil and exclusively religious. That’s often why they like it."
 
The Catholic Latin Patriarchate in Jerusalem has shown witness to unity in Christian worship by largely adopting the Orthodox and Julian date for Easter "so that all the Christians of the apostolic jurisdictions there show unity in a place where unity is crucial to their survival," Fr. Barbour pointed out. "This is a deep reason to consider a common date of Easter, suggested already by the Second Vatican Council 60 years ago."

To celebrate as one

Because of the different calendars and methods of calculating the date, Catholic Easter and Orthodox Easter coincide less than one-third of the time. Some years the two Easters are separated by more than a month.

During the 21st century, if the present systems were to continue, Catholics and the Orthodox churches would celebrate Christ’s Resurrection on the same day just 31 times. The most recent common Easter occurred on April 16, 2017, and the next will be April 20, 2025. After that, barring a Catholic-Orthodox agreement, the next common Easter won’t take place until April 16, 2028.