Many people wonder about how the date of the feast of Resurrection is calculated? Why is it sometimes different from other Christian denominations? And what is the history behind this difference?

I am pleased to answer these questions in this article:

Until the beginning of the third century AD, many Churches in the world celebrated the feast of Resurrection (AKA Easter) on different dates, even though they were united in the faith. Hence, there was a need to unify the date of celebrating Easter according to an accurate calculation to be followed by all churches. Since the Copts were experienced in astronomy, they took the initiative in doing so in the third century. This was done by  Ptolomy of Farma, an Egyptian astronomer form the town of Farma near Port Said [Egypt Northeast] during the papacy of Pope Demetrius of Alexandria [AKA Demetrius the vinedresser], the 12th pope (189-232). The calculation was named after the pope “Hisab Alkarma” [The calculation of the vine] or the “Calculation of EPACTE” which means “the moon cycle at the beginning of the Coptic month of Tout”

This calculation unified the date of celebrating the Resurrection feast throughout the world.  All bishops of Rome, Antioch, and Jerusalem indeed agreed to the calculation when Pope Demetrius of Alexandria wrote to them in that regard. When the council of Nicaea convened in 325 AD,  it accepted this arrangement and all Christian churches adhered to it till 1582 as we will explain later.

This Calculation takes into account the following conditions for the celebration of the Resurrection:

1. It must take place on a Sunday because the Resurrection of the Lord was in fact on a Sunday
2. It must take place after the Vernal Equinox (March 21)
3. It must take place after the Passover of the Jews because Resurrection took place after the Jewish Passover.

Since the Jewish Passover comes on the 14th day of Nissan, the first month of the Hebrew calendar (lunar), therefore Easter must come after the full moon in the second half of the lunar Hebrew month. The Jewish Passover is tied to the harvest, according to God’s word to Moses (Lev. 23:4-12), for the Jews, it falls between April and May (Solar calendar). Therefore, it was necessary to devise a calendar that combines the solar and lunar calendars [Lunisolar] such that Easter falls between April and May. This way Easter wouldn’t be celebrated before the first week of April or after the first week of May.

There is too many mathematical details to the calculation but in simple terms it is a cycle that repeats itself every 19 years. Based on the calculation, Easter doesn’t come before April 4 or after May 8.

Resurrection feast continued to be celebrated on the same date for all Christian denomination throughout the world until 1582 when Pope Gregory the 13th of Rome changed this arrangement. The result was that the Resurrection feast for western churches takes place after the full moon immediately following the vernal equinox regardless of the date of the Jewish Passover, despite that the Resurrection of Christ took place after the Passover of the Jews according to the four Gospels. Consequently, western Easter sometimes coincides with the Eastern Churches , and other times it comes earlier by one to five weeks at most, but never comes after the eastern celebration of the feast.

It is worth mentioning that the Protestants did not like the Catholic Church modification of the date of celebrating Easter. They continued to follow the eastern calendar until 1775 AD but due to the increasing influence of the west, they moved from the eastern calendar to the Gregorian.

The purpose of the EPACTE calculation is to determine the date of celebrating the Resurrection feast, and based upon it, the following feasts can be determined: Ascension, Pentecost, Fast of the Apostles, etc.