Q. How is the Easter date chosen, and why does it change from year to year?

A. This year, the date of Easter is the same for both the Western (Catholic and Protestant) and Eastern (Orthodox) churches, but that is not always the case. Also, this year Easter falls on one of the eight days of Passover, but that is not always the case. How are these dates chosen, and why do they change from year to year?

Each of these groups uses a different calendar. The Jewish calendar is based on the lunar cycle, and has 12 months in most years, with an additional month periodically added to the year in order to keep the calendar in rough accord with the solar cycle. Passover always begins at sundown of the 14th day of the month of Nissan and lasts 7 days in Israel and 8 days outside Israel. Because the months of the Jewish calendar do not correspond exactly with the months of the modern solar calendar, the date of Jewish holydays changes from year to year, but they remain in the same season of the year. Passover is always a spring holy day.

Early Christianity sometimes celebrated Easter on 15th of Nissan. However, since Christ rose on a Sunday (the day after the Sabbath of the Jews) and Christians wished to celebrate Easter on Sunday, a new method of calculation was needed. For a time the date of Easter was simply decreed by the bishop of the nearest large city.

The first Ecumenical Council was held at Nicea in present-day Turkey in the year 325. It decreed that Easter would be celebrated on the Sunday following the first full moon that occurred after the spring equinox. This retained a lunar connection as a sort of 'memory' of the Jewish calendar system, and ensured that the feast would be on a Sunday. Because lunar phases occur independently of the solar year, this means that there is a 'window' of several weeks during which Easter may be celebrated. By this reckoning, in our calendar, Easter must occur between March 22 and April 25.

The Council of Nicea is accepted by both western and eastern Christianity, and the two celebrated Easter on the same date until the 1500s. However, in the 16th century the Catholic world moved from the Gregorian calendar (named after Pope Gregory the Great) to the Julian calendar (named after Pope Julius II), which reflected more advanced scientific understanding of the length of the solar year (it added leap years, and adjusted the date by 11 days in order to recalculate the solar calendar year). The Julian calendar was gradually accepted by Protestant countries, and so the west agreed to celebrate Easter on the same date. The Orthodox churches, however, have never accepted the Julian calendar, and therefore the date of Easter is usually different for the Orthodox church on the one hand, and the Catholic and Protestant churches on the other.

The overlap of Passover, Easter, and Orthodox Easter, occurs when these three different calendar systems happen to coincide in a particular year.