Easter, the day Christians commemorate the resurrection of Jesus Christ, is known as a "moveable feast" because the dates are always different. Likewise, the dates for holidays leading up to it—Ash Wednesday, Palm Sunday, Good Friday—shift accordingly. Easter can fall on any Sunday between March 22 and April 25, according to the Gregorian calendar. Introduced by Pope Gregory XIII in 1582, it is known as the system that Western Christian churches use to calculate the date. But this method is not in sync with the "lunar" calendar. One explanation, from the Explanatory Supplement to The Astronomical Almanac:
In the Gregorian calendar, the date of Easter is defined to occur on the Sunday following the ecclesiastical Full Moon that falls on or next after March 21. This should not be confused with the popular notion that Easter is the first Sunday after the first Full Moon following the vernal equinox. In the first place, the vernal equinox does not necessarily occur on March 21. In addition, the ecclesiastical Full Moon is not the astronomical Full Moon -- it is based on tables that do not take into account the full complexity of lunar motion. As a result, the date of an ecclesiastical Full Moon may differ from that of the true Full Moon.
Eastern Orthodox churches, on the other hand, follow the solar, Julian calendar, adopted by Julius Caesar in 45 BC. This timetable has been 13 days ahead of the Gregorian calendar, so that's why “the Orthodox Easter celebration usually occurs later than that celebrated by Protestants and Roman Catholic s,” according to Encyclopedia Britannica. “The Orthodox tradition prohibits Easter from being celebrated before or at the same time as Passover .”
For decades, storekeepers, even religious leaders, have called for a set day for the holiday. Last spring, Pope Francis said he was open to the idea of a fixed date for Easter, and earlier this year, the Archbishop of Canterbury, Justin Welby, announced that he was working with other Christian churches to decide on one in the next "five to 10 years" — recommending it fall on either the second or third Sunday of April.