Who Determines the Date of Pascha in the Orthodox Churches? And How Is It Calculated?
Answered by Priest Gleb Bobkov:
The rule for determining the date of Pascha (Easter) can be summarized as follows: “Pascha is celebrated on the first Sunday after the first full moon that occurs after the spring equinox.”
The Alphabetical Syntagma of Matthew Vlastar on the date of Pascha states: “Concerning our Pascha, attention must be paid to four determinations, two of which are found in the Apostolic rule, and two originate from unwritten tradition:
First — we must celebrate Pascha after the spring equinox;
Second — we must not celebrate it on the same day as the Jews;
Third — we must celebrate it not merely after the equinox, but after the first full moon that follows the equinox;
Fourth — after that full moon, Pascha must fall only on the first day of the week (i.e., Sunday).”
The Christians of Asia Minor strictly adhered to the custom of celebrating Pascha on the 14th of Nisan, basing this practice on the authority of the Apostle John the Theologian.
In the Western Church, a different custom developed: to celebrate Pascha on the first Sunday following the Jewish Passover, which was itself calculated as the full moon after the spring equinox.
In the year 155, Polycarp of Smyrna visited the Pope of Rome in an attempt to reach an agreement on the common observance of Pascha, but no consensus was reached. Later, between 190 and 192, Pope Victor of Rome, at synods held in Palestine, Pontus, Gaul, Alexandria, and Corinth, insisted that the Christians of Asia Minor abandon their practice. He also demanded that other churches break communion with them.
St. Irenaeus of Lyons opposed this move to excommunicate the Christians of Asia Minor, arguing that disagreements on formal points should not endanger the unity of the Church.
At the First Ecumenical Council in 325 AD, it was resolved that the date of Pascha should be unified among the Churches. The Council condemned the practice of using the Jewish date, which at times could fall before the spring equinox: “When the question arose regarding the most holy day of Pascha, it was decided by universal agreement that this feast should be observed by all on the same day everywhere… And indeed, it appeared highly improper that in celebrating this most sacred feast, we should follow the custom of the Jews…”
It was therefore determined that Pascha should be celebrated on the first Sunday after the first full moon that occurs on or after the spring equinox.
The Bishop of Alexandria was entrusted with calculating this date and informing Rome in advance, to ensure a unified celebration of the feast.
In Alexandria, Paschal tables — a kind of Paschal calendar — were created to allow the determination of the date of Pascha far into the future. These were based on a 19-year lunar-solar cycle, which was later refined into what became known as the Great Indiction. Over the course of 19 × 28 = 532 years, the calculated phases of the Moon and the days of the week align with the same dates on the calendar. Thus, every 532 years, the dates of Pascha repeat in full. This period came to be known as the Great Indiction.