Life of Saint Irene https://orthochristian.com/96000.html
One of those questions that come up all the time
Every year around Easter time, someone is bound to ask us, "Why is your (Orthodox) Easter later than ours (i.e. the Roman Catholic and Protestant Easter)?" Of course, on some years, the date is the same, but most years it is not. Why the difference?
Our original Orthodox term for "Easter" is "Pascha," which is the Greek form of a Hebrew word, "Pesach," meaning "Passover." The Resurrection of Christ is the Christian Passover, fulfilling, surpassing, and REPLACING the Passover of the Old Testament. So the first thing to note is that the Christian Pascha and the Jewish Passover are linked:
The Date of Passover
In the law of Moses, the date of Passover is set as the 14th day of Nisan, the lunar month which falls at the beginning of spring. Since it is a lunar date, it is movable - it does not fall at the same time year to year, but moves around a bit, from late March to late April.
The Orthodox Paschalion
During the first three centuries of the Church's history, the Fathers gradually worked out a formula for determining the date of Pascha every year. This formula, or method, for establishing the date of Pascha is called the Paschalion. Because the city of Alexandria was then the leading center of mathematical and astronomical studies, the Patriarchate of Alexandria was entrusted with determining the date of Pascha every year and sending an encyclical to all the Churches to make sure that everyone celebrated on the same date. The "Alexandrine" Paschalion was approved by the First Ecumenical Council in 325 AD and has been in use ever since.
What is the formula?
There are mathematical formulas that form the precise method, but in layman's terms, this is roughly the idea:
The First Sunday of Pascha is the:
Why do the Catholics and Protestants have a different date almost every year?
During the Middle Ages, the Popes decided that they did not have to observe the prohibition against celebrating Pascha at the same time as the Jews. Therefore, there began to be a frequent one-week difference between the Orthodox Pascha and the Latin Pascha.
Then, in the 16th century, Pope Gregory XIII changed the calendar in such a way that the New spring equinox (March 20) now occurred well before the Julian calendar March 20. At first the difference was ten days, but now it is 13 days. So more and more often, as the centuries go on, there will occasionally be an entire lunar cycle falling between the competing March 20th dates, Old and New. When this occurs, there is a five week difference! When this happens, the Roman Catholic Easter is celebrated well before the Jewish Passover even begins.
PS: Pascha NEVER falls in May. Even when it says "May" according to the New Calendar, the date of Pascha is never later than the end of April. For example, in 2016, the date of Pascha is "May 1" on the New Calendar, but it is REALLY April 18 on the calendar the Church actually uses to determine Her feast days, the "Julian" or "Old" Calendar.
PPS: Remember to tell your New Calendar friends that, for the Paschalion, they are still on the Old Calendar, though their leaders usually obscure the fact.