To make understood as to how Pascha, or Easter is Celebrated in the Eastern Orthodox Church, I'll leave the following teaching.http://stgeorgecathedral.org/explanations/easter.html
James C. Rouman, M.D.
All of Christendom comes together every four years to celebrate the most important period of the ecclesiastical calendar, namely Easter and Holy Week, which leads people to wonder why it is not so during the other three years. Though it would seem that all Christians would want to be united in the commemoration of our Lord's Passion, we of the East are sometimes perceived as renegades, stubbornly preventing this from ever happening. It is important, therefore, that we understand the historical reason for our position, which is clearly rooted in history and in scripture.
In the year A.D. 325, the Roman Emperor Constantine assembled the Council of Nicea (the First Ecumenical Council of the ancient and undivided church) for the purpose of defining important theological dogmas of faith and to determine pragmatic issues concerning the Church at the time. Along with other canons established at the council, the assembled Fathers arrived at a formula for determining the date of Easter, and ruled that it should be celebrated on the first Sunday after the first full moon of the Spring (vernal) Equinox. Furthermore, they decreed that in accordance with sacred tradition, which is normative in this matter, Easter must always follow or occur during the Jewish Passover. And so Easter was observed in the entire Christian world until 1582 when the Gregorian calendar replaced the Julian calendar in much of the West.
Although many Orthodox churches today follow the Gregorian calendar for observing other feast days such as Christmas, all of Orthodoxy continues to observe Easter according to the canonical precepts of the First Ecumenical Council, using instead the Julian calendar in their calculations. In this way, the time of Easter always falls during or after the Jewish Passover, but never before in order to conform to New Testament events as we believe them to have taken place.
When Pope Gregory and his scholars established his more scientifically accurate calendar in 1582, the Roman Church, from which Protestantism derived, continued using the Nicene formula, but could no longer with this new calendar include the Jewish factor in their determination of the Christian Passover, called Pascha and better known as Easter. As a result, with the Gregorian calendar Western Christians, more often than not, observe Easter before the Jewish Passover, which flies in the face of New Testament accuracy, i.e. that (1) Christ was a Jew; (2) The Jews at that time were celebrating the Passover; (3) The Last Supper was probably a "blessing meal" similar to a Seder; and (4) It was Springtime and the moon was full.
Whereas all Christians prior to 1582 celebrated Easter together, we Orthodox Christians must not feel a sense of embarrassment observing Easter as it was originally intended by the Church Fathers, although now out of sync with our Western brethren. Rather, we should be comfortable in the knowledge that persons of the Jewish faith will have completed their Passover observances before we commemorate the crucifixion and resurrection of Christ----historical events that are so clearly described in scripture.
Ask yourselves, then. Wouldn't it make more sense if all of Christendom celebrated Easter when we do?