Great Lent for the Orthodox begins on Clean Monday rather than the West’s Ash Wednesday. So, if the feast of Pascha or Easter falls on the same day for both the Orthodox Church and the West, Lent begins two days earlier for the Orthodox.
However, you might first wonder why Pascha often falls on different days. In the early Church, different Christian communities observed the resurrection of the Lord according to various methods. The Churches of some regions celebrated the feast according to the Jewish lunar calendar, while other Churches insisted that the feast always fall on the first day of the week—Sunday. At the first Council at Nicea in A.D. 325, the assembled bishops agreed on a standard and universal formula for dating Pascha. Pascha is to be celebrated on the first Sunday after the first full moon after the vernal equinox, as long as the Jewish passover has already occurred. However, discrepancies continued because different regions observed different dates for the vernal equinox and had different calculation methods for predicting the lunar cycles. Eventually, the Churches adopted standard calculation methods and a standard date for the vernal equinox—March 21—instead of relying on the actual astronomical phases of the moon and vernal equinox that falls on different days around March 21, depending on the year. For all Christians to celebrate Pascha on the same day, without precise astronomical instruments and easy long-distance communication, it was practically necessary to adopt such uniform standards.
Though the council issued the new universal system, it still took several centuries for every Christian region to accept it. After universal acceptance, Pascha was celebrated on the same day until the Roman Pope Gregory XIII reformed the calendar in A.D. 1582. The Orthodox Church never accepted Rome’s unilateral calendar change. Since then, the date of March 21 on the Julian and Gregorian calendars has differed. Such explains the frequent discrepancy of date for the celebration of Pascha. In this year of our Lord 2009, for example, Gregorian Pascha falls on April 12, while Orthodox Pascha falls one week later on April 19.
Tomorrow, I shall explain why the Lenten fast has a different timetable for the East and for the West in “Why Clean Monday?”