In 2025 will occur the 1700th anniversary of the Council of Nicaea, the first Ecumenical Council, summoned by the Emperor Constantine in 325. This Council gave us the Creed — although it was slightly expanded at the First Council of Constantinople in 381 — that unites all Christians today. This Council also established the common formula for the date of Pascha, the most important feast for all Christians: It is to be observed on the first Sunday after the first full moon of spring.
Over the centuries, East and West came to differ over how to determine which Sunday is the first Sunday after the first full moon of spring, but in this year 2025 both East and West will celebrate Pascha on the same day. Because of this happy coincidence on so significant an anniversary, many Catholic, Orthodox, and Oriental Orthodox Christians see this year as a fitting opportunity to achieve new agreement regarding the date of Pascha. In the following, I'll briefly summarize how the disagreement arose, the most prominent proposal for its resolution, recent developments, and a proposal of my own.
The disagreement between East and West over the date of Pascha arises from a question implicit in the formula of Nicaea itself: When does spring begin? For purposes of the ecclesiastical calendar and for most of their history, the Christian East and the Christian West have agreed on the date of March 21 as the start of spring. However, as centuries passed from the time of Nicaea, the Church observed that the calendar and the seasons of the year were slowly drifting out of sync. This prompted a new and puzzling question for the Church: Which day is March 21?
The Council of Trent sought to restore date of March 21 to the day with which it would have been identified at the time of Nicaea. This led to the promulgation of the Gregorian Calendar in 1582. While the Julian Calendar observes a leap day every four years without exception, the Gregorian observes a leap day every four years but skips three of them every four centuries. The Gregorian Calendar thereby approximates more closely the length of the solar year and reduces the drift of seasons. Furthermore, to correct for the drift of the foregoing centuries, the dates were skipped that fell between October 4 and October 15 in 1582. (This led to an unusual ambiguity regarding the date of the death of a prominent Western saint, St. Teresa of Avila: She died either in the late evening of October 4 or the early morning of October 15.)
The difference in leap years and the skipped dates of 1582 account for the divergence between the Old Julian and Gregorian Calendars that we see today. The situation was made slightly more complicated by the transition of several Local Churches, now including the Orthodox Church of Ukraine and the Ukrainian Greek Catholic Church, to the New Julian Calendar. The New Julian Calendar corrected the drift of seasons by re-aligning dates with the Gregorian Calendar, but by preserving the Julian Calendar paschal calculations it effectively shifted the start of spring from March 21 to April 3.
Nowadays, proponents of the Gregorian Calendar claim that it is more faithful to the formula of Nicaea than the Julian Calendars. However, neither the Gregorian Calendar nor the Julian Calendars determine the date of Pascha with precision in relation to the astronomical phenomena named in the formula of Nicaea: the full moon and the start of spring. This is because both the Julian and the Gregorian Calendars not only assign the start of spring to a fixed date but also merely estimate the length of lunar cycles. Therefore, they often fail to reflect the lunar phenomena visible in the night sky.
Consequently, at conference in Aleppo, Syria in 1997, the World Council of Churches sought to address both issues when it proposed a new and more precise method for determining the date of Pascha. Rather than treating the start of spring as a mere fixed date and rather than merely estimating the length of lunar cycles, the Church could instead use the most advanced technology available to track the precise moments of the vernal equinox, i.e. the astronomical beginning of spring, and of the subsequent full moon. In both cases, the precise moment depends upon where you're standing on the earth. So the conference recommended making use of the meridian (longitude) of Jerusalem. For purposes of the ecclesiastical calendar, spring would begin at the exact moment that it begins astronomically in Jerusalem, and the full moon would occur at the exact moment when it is full over Jerusalem. This came to be called the "Aleppo Method" and was proposed as a new way to determine the date of Pascha more faithful to the formula of Nicaea than any method yet employed.
Although widely praised, the Aleppo Method has not yet been adopted by any Local Church. It finds support among scholars in both East and West. Yet, others oppose it: Some argue that a change in paschal calculations poses a grave inconvenience for intra-Orthodox or inter-Orthodox unity or for the unity of Western Christianity. Some have held the strange position that the Church is obligated to retain her traditional estimations of the time of the vernal equinox and length of lunar cycles. Still others have discerned in the holy canons an additional criterion for determining the date of Pascha: that it must occur after the Jewish celebration of Passover. Like the Old Julian Calendar, the Jewish Calendar also suffers from seasonal drift. So while the Julian Calendar paschal calculations always put Pascha after the Jewish Passover, a correction according to the astronomical phenomena named in the formula of Nicaea will put Pascha before the Jews celebrate Passover in some years.
In recent years, the Ecumenical Patriarchate of Constantinople seemed to favor adopting the Aleppo Method. But beginning with the Eastern Orthodox-Oriental Orthodox meeting near Alexandria in September 2024 and again at the World Policy Conference in Abu Dhabi in December, Patriarch Bartholomew has called for Western Christians to return to the Julian Calendar paschal calculations. Pope Francis has repeatedly hinted that he is open to the proposal. Yet the Vatican has given no other indication that the Catholic Church will adopt it. Perhaps we will see more on the topic as Pascha approaches or during further reflection on the Council of Nicaea in May, June, and July.
To conclude, I will share a compromise that I have proposed in Rome: that the Catholic Church indeed accept the invitation of the Orthodox Churches to adopt immediately the Julian Calendar paschal calculations, on the one hand, and, on the other hand, that the Catholic Church also propose a future synod to consider together with all her ecumenical partners the adoption of the Aleppo Method for determining the date of Pascha. In order to answer those who oppose celebrating Pascha before the Jewish Passover, an invitation to consider calendar reform and to attend the synod as guests could be extended to the Jews as well; although the Jewish calendar is an internal matter for the Jews, Christians and Jews together suffer from calendar drift, and nothing prevents our cooperation in this respect.
In this century, there is one and only one year in which the Aleppo Method agrees with the Julian Calendar on the date of Pascha and not with the Gregorian Calendar — the year 2049. In other words, the Julian Calendar is more astronomically accurate than the Gregorian Calendar in the year 2049. Therefore, if a change is to be made, 2049 would seem to be a fitting year to retire the Julian Calendar paschal calculations and to adopt a new method for determining the date of Pascha more faithful to the paschal formula of the Council of Nicaea.