The Orthodox Church calculates the date for Holy Pascha (Easter) according to the decision of the First Ecumenical Council, held at Nicea (near Constantinople) in the year 325 A.D.
Among other matters, including the formulation of what we call the “Nicean Creed,” that Council had been called to establish a formula for calculating the date of Pascha each year. Their decision was accepted by the entire Christian Church (including Rome) and is as follows:
1) The Feast of the Resurrection must be celebrated the first Sunday following the first full moon after the vernal (spring) equinox. If the full moon occurs on a Sunday, Pascha is then to be celebrated on the following Sunday.
2) The Resurrection must always be celebrated after the Jewish Passover. This is to ensure the proper historical sequence of events as recorded in Holy Scripture, and to make a thorough distinction between the two passovers. For the Jews, the Passover is their liberation from slavery; for Christians, it is the liberation of man from sin.
Pascha sometimes falls on the same date for both Orthodox and western Christians. Only in the year 1582 did the western church begin celebrating Pascha/Easter on days other than the Orthodox observance.