Q. Why do Orthodox Christians celebrate Easter on a different day from Catholics?
A. The day designated for annually celebrating our Lord’s resurrection varied within the early Church, a situation that sometimes led to controversy. The fathers of the First Council of Nicaea (a.d. 325) recognized that the unity of Catholic faith throughout the world would be better reflected by a consistency in the celebration of this most important of Christian feasts. So they standardized the method for determining its date.
Unlike Christmas, which always occurs on a set date (December 25), Easter was established as a moveable feast, dependent on the shifting relations each year between the cycles of the moon and the sun. The Nicene fathers declared that Easter would fall on the first Sunday after the first full moon to occur after the vernal equinox.
The vernal equinox is the time each spring when the sun crosses the plane of the earth’s equator, so that night and day are of approximately equal length all over the world. It usually falls on March 21. Thus Easter never occurs before March 22 or after April 25.
We should note that this not a precise statement of the actual Church rules for determining the date. The full moon involved in calculation is not the astronomical full moon, but rather an “ecclesiastical moon” determined from rather complicated tables developed by the Church. Nevertheless, the ecclesiastical moon keeps in step, more or less, with the astronomical moon.
If the Nicene fathers standardized the celebration of Easter, how is it that Eastern Orthodox Christians usually celebrate on a day different from Catholics and most Protestants? The ancient Nicene rule for calculation is in fact still followed essentially by all these Christian communions. But the basic calendar used by the Orthodox churches is itself no longer used in the West.
At the time of Nicaea, the Roman world was using the Julian calendar, so called because it had been introduced by Julius Caesar about 46 b.c. However, the imprecision of this calendar allowed the true (seasonal) year to move away from the calendar year over a period of centuries.
To solve this problem, in 1582 Pope Gregory XIII made adjustments to the Julian calendar to make it correspond more closely to the true length of the solar year. The new arrangement was called the Gregorian calendar.
Catholic countries adopted the Gregorian calendar in 1582, but by that time a discrepancy of 10 days had accumulated between it and the Julian calendar. So the extra 10 days were eliminated by having the date jump that year straight from October 4 to October 15.
By the early twentieth century, most countries had adopted the Gregorian calendar, at least for secular purposes. Nevertheless, the Eastern Orthodox churches, long separated from papal leadership, continued to use the Julian calendar to calculate the date of Easter. That calendar now runs 13 days after the Gregorian one.
As a result, Catholic and Orthodox Christians usually end up observing our Lord’s resurrection — not to mention the Lenten season and Holy Week preceding it — on different dates.
As an interesting side note: After five centuries, the Protestant denominations that have separated from the Catholic Church still depend on the Catholic Church’s calendar and protocol to determine which day they will celebrate as Easter.