How the date of Orthodox Easter is calculated

Orthodox Easter (Pascha) has no fixed date. In 2026 it fell on 12 April; in 2027 it falls on 2 May, almost three weeks later. Since a whole group of name days is counted from Easter, the calendar works out its date afresh every year. This guide explains the Church's rule, the calculation the site uses and why Orthodox and Western Easter usually differ.

The rule from Nicaea

According to OrthodoxWiki, the rules for dating Pascha, known as the Paschalion, are attributed to the First Ecumenical Council, which the Emperor Constantine the Great summoned to Nicaea in 325. The decision was not written down as a canon. What survives is the council's letter to the Church of Alexandria and a letter of Constantine, who wanted the feast kept everywhere on one and the same day.

Before Nicaea, OrthodoxWiki explains, many churches relied on Jewish informants and kept Pascha on the third Sunday of the month of Nisan. By the third century some objected that this occasionally put the feast before the spring equinox, and Peter of Alexandria wrote that it belonged after the equinox. At Nicaea the bishops sided with Alexandria, which worked out the date by its own calculation, and set aside the practice of Antioch, which followed the Jewish reckoning. The older custom of keeping Pascha on the Jewish date of 14 Nisan was also rejected.

OrthodoxWiki sums up the resulting rule in four points:

• Pascha comes after the spring (vernal) equinox.
• It follows the first full moon of spring, but never falls on the day of that full moon.
• It is always a Sunday.
• It does not depend on the date of the Jewish Passover.

A full moon on paper

The Church does not look at the sky to apply this rule. It uses tables that go back to Alexandrian models of the third and fourth centuries, with two simplifications built in:

• The equinox is fixed on 21 March of the Julian calendar.
• The full moons are ecclesiastical full moons, taken from a 19-year cycle after which the phases of the moon return to nearly the same dates. Easter is the first Sunday after the first such full moon that falls on or after 21 March.

Both simplifications have drifted. A Julian year lasts 365.25 days, a little longer than the solar year of about 365.2422 days, so the calendar slowly moves away from the seasons. OrthodoxWiki notes that the Church's full moons now come three to five days after the real ones, and 21 March of the Julian calendar is today 3 April on the civil calendar.

Why there are two Easters

In 1582 the Roman Catholic Church adopted the Gregorian reform, which removed the ten days the equinox had drifted, and began to calculate Easter with new tables. The Eastern Churches objected, among other reasons because the new tables could put Pascha on the very day of the spring full moon instead of after it, and a council in Constantinople in 1583 forbade both the new calendar and the new Paschalion.

In 1923 a congress of Orthodox Churches in Constantinople considered the Revised Julian calendar, the “new calendar” that the Church of Greece now follows for fixed feasts, and also proposed an astronomical rule for Easter. According to OrthodoxWiki, every Orthodox Church rejected that rule and still calculates Pascha with the Julian calendar; the exceptions are the Churches of Finland and Estonia, which use the Gregorian Paschalion. That is why Orthodox Christians on the new and on the old calendar keep Easter on the same day; see the guide to the old and new calendar.

The calculation this site uses

This calendar computes Easter with the method for the Julian calendar published by the astronomer Jean Meeus in Astronomical Algorithms (chapter 8). It takes five steps of whole-number arithmetic, where “mod” means the remainder of a division:

1. a = year mod 4, b = year mod 7, c = year mod 19.
2. d = (19 × c + 15) mod 30.
3. e = (2 × a + 4 × b − d + 34) mod 7.
4. month = (d + e + 114) ÷ 31, dropping the remainder; day = ((d + e + 114) mod 31) + 1. This is Easter in the Julian calendar.
5. Add the gap between the two calendars to get the civil date: 13 days for the years 1900–2099.

c is the year's place in the 19-year lunar cycle, and d, which can be anything from 0 to 29, counts the days from 21 March to the ecclesiastical full moon. a and b follow the days of the week through the four-year cycle of leap years and the seven-day week, and e (0 to 6) is the number of days still needed to reach a Sunday. Put together, Easter falls d + e days after 22 March in the Julian calendar.

Example, 2027: a = 3, b = 4 and c = 13, so d = 262 mod 30 = 22 and e = 34 mod 7 = 6. Then d + e + 114 = 142, which gives month 4 and day 19: 19 April in the Julian calendar, which is 2 May on the civil calendar.

The same arithmetic reproduces the example OrthodoxWiki gives for 2006: 10 April in the Julian calendar, 23 April in the civil one.

The earliest and latest possible date

Since d is at most 29 and e at most 6, Julian Easter always falls between 22 March and 25 April. Adding 13 days, Orthodox Easter in the years 1900–2099 can fall no earlier than 4 April and no later than 8 May. Run over those two centuries, the site's code finds the earliest date in 1915 and 2010, and the latest in 1983 and 2078.

The gap between the calendars grows by one day whenever the Julian calendar has a 29 February that the Gregorian one skips. The next such year is 2100, so from then on 14 days are added: Easter 2100 falls on 2 May, and until 2199 the possible dates run from 5 April to 9 May.

How far apart the two Easters are

Under the present rules, OrthodoxWiki notes, Orthodox and Western Easter fall on the same day or one, four or five weeks apart. The big gaps come in five years of every 19-year cycle, when the two calculations settle on different full moons. In every year from 2000 to 2040 the Orthodox date is either the same as the Western one or later.

A few examples: in 2025 both Churches celebrated on 20 April; in 2021 Orthodox Easter (2 May) came four weeks after the Western one (4 April); and in 2027 the gap is five weeks, 28 March against 2 May.

Easter in the coming years

Orthodox dates calculated with the site's code; the Western dates, for comparison, follow the Gregorian method in the same chapter of Meeus.

• 2027: 2 May (Western: 28 March)
• 2028: 16 April (same day)
• 2029: 8 April (Western: 1 April)
• 2030: 28 April (Western: 21 April)
• 2031: 13 April (same day)
• 2032: 2 May (Western: 28 March)
• 2033: 24 April (Western: 17 April)
• 2034: 9 April (same day)
• 2035: 29 April (Western: 25 March)
• 2036: 20 April (Western: 13 April)

Name days that move with Easter

Every movable name day lies a fixed number of days before or after Easter, so once Easter is known the rest follows. For 2027, with Easter on 2 May:

• Saturday 20 March, Saturday of Saint Theodore (43 days before Easter): Thodoris, and Theodora as a second date.
• Saturday 24 April, Lazarus Saturday: Lazaros and Lazaria.
• Sunday 25 April, Palm Sunday: Vaios, Vaia and Dafni.
• Sunday 2 May, Easter: Lambros, Lamprini, Anestis, Anastasios, Anastasia, Paschalis and Paschalina.
• Monday 3 May: Giorgos and Georgia, because 23 April 2027 falls in Lent and the feast of Saint George moves to Easter Monday.
• Tuesday 4 May: Rafail (Raphael), with Saints Raphael, Nicholas and Irene, and Markos, whose 25 April falls on Palm Sunday, so his feast moves to Easter Tuesday.
• Friday 7 May, the Life-giving Spring: Zoi.
• Sunday 9 May, Thomas Sunday: Thomas and Thomais.
• Monday 21 June, Monday of the Holy Spirit: Triada.
• Sunday 27 June, All Saints' Sunday: names without a saint of their own, such as Ilektra and Orfeas.

The guide to movable name days explains each of these feasts, and why Saint George sometimes moves after Easter lists the years in which his feast is transferred. Every name page shows its date for the coming years.