Twelve Great Feasts were celebrated by the Eastern church, these feast days commemorate events in the lives of Christ and the Virgin. The feasts are:
Koimesis commemorates the death of Dormition (sleep) of the Virgin, and is known in the West as the Assumption (which, however, stresses Mary's bodily resurrection rather than her death). The Byzantine emperor Maurice (582-602) fixed the feast day at August 15, where it has remained.
Dates in the Julian Calendar assume a year beginning January 1. The date was determined using the formula from the Council of Nice. I provide the backdated Gregorian dates just for usefulness. Prior to AD 1054, there was no separate Eastern Church. After Orthodox and Catholic churches split in 1054, they still used the same formula for Easter, until the 1583 when the Gregorian calendar (and a new formula to determine the date of Easter) were adopted throughout much of Europe.
Between AD 326 and AD 1582, Christianity determined Easter using an algorithm approved by a Church Council in AD 325, with the equinox defined as March 20 (the date in the Julian calendar of the equinox in AD 325). From AD 1054 (when the Orthodox and Catholic Churches split) through AD 1582 both the Catholic and Orthodox Churches celebrated Easter on the same date, still using the algorithm from AD 325. The Julian Calendar was used by the European (and Christian) communities until the Gregorian reform of 1582.
Since AD 1582 October (when the Gregorian Calendar was adopted by much of Catholic Europe), the Orthodox Easter usually falls on dates different than the Western Christian Easter, although apparently the Churches are discussing using the same formula to determine Easter - probably a formula different than that currently used by either Church.
The Orthodox Easter is determined in the Julian Calendar, and is never before or on the Jewish Feast of Passover (15 Nisan in the Hebrew Calendar).
The Eastern Easter has a cycle that (in the Julian Calendar) repeats itself every 532 (19x28) years (since the Julian Solar calendar repeats every 28 years and the Metonic Lunar cycle is 19 years). Because of the strict concordance with the lunar cycle, the Eastern Easter occurs only after Passover. The Gregorian Easter, on the other hand does not track Passover. For example: in 1997, Passover is 22 April; Western Easter is three weeks EARLIER (30 March) and the Eastern Easter is the Sunday following Passover (27 April). The following algorithm is derived by the German mathematician Gauss. This algorithm calculates the number of days AFTER March 21 (Julian) that Easter occurs (Note: It is a much simpler calculation than the Western Easter).
RMD(x,y) = remainder when x is divided by y.
R1 = RMD(Year,19)
R2 = RMD(Year,4)
R3 = RMD(Year,7)
RA = 19*R1+16
R4 = RMD(RA,30)
RB = 2*R2+4*R3+6*R4
R5 = RMD(RB,7)
RC = R4+R5
The number RC ranges from 1 to 35 which corresponds to March 22 to April 25 in the Julian Calendar (currently April 4 to May 8 on the Gregorian). The Julian Calendar is now 13 days behind the Gregorian, and will be until March 1, 2100 when it will be 14 days behind the Gregorian Calendar.
Lastly, what follows is a brief tabular listing of where the important liturgical date fall, given that Easter has been calculated for a given date.
| Days before Easter | Days after Easter | ||
| Triodon | 70 | Ascension | 39 |
| Sat. of Souls | 57 | Sat. of Souls | 48 |
| Meat Fare | 56 | Pentecost | 49 |
| 2nd Sat. of Souls | 50 | All Saints | 56 |
| Lent Begins | 48 | ||
| St. Theodore | 43 | ||
| Sun. of Orthodoxy | 42 | ||
| Sat. of Lazarus | 8 | ||
| Palm Sunday | 7 | ||
| Good Friday | 2 |
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Last modified: Mon Dec 14, 1998