The date for Easter changes annually because of differences between lunar and solar cycles. This creates a sequence of "movable" observances. Based on guidance established by the Council of Nicaea in 325 AD/CE, the Easter celebration occurs on the first Sunday that follows the first new moon after the spring equinox, which the ancient council established as March 21 (an astronomical approximation).
The Julian Calendar used at that time had been adopted in 45 BC/BCE by Julius Caesar. It was based on a year length of 365.25 days. Because the true length of a solar year is 365.2422 days (11 minutes and 14 seconds shorter than the calendar's calculations), the astronomical spring equinox occurred slightly earlier than predicted by the calendar. By the mid sixteenth century, a difference of 13 days had accrued.
Pope Gregory advocated for calendar reform. The Gregorian Calendar, adopted by the Roman Catholic Church in 1582, subtracted 13 days to realign the calendar's spring equinox more closely to the astronomical equinox, and it adjusted the calculations for leap years. Today, the Gregorian Calendar is the most commonly used civil calendar.
As a result of applying the formula for calculating the date on which Easter will be observed to different calendar systems, Western and Eastern Easter celebrations often fall on different dates. Western traditions, which include Roman Catholic and Protestant communities, follow the Gregorian calendar, and Easter can occur as early as March 22 or as late as April 25. For Eastern branches of Christianity, which include Orthodox churches, the observation of Easter can occur as early as April 3 or as late as May 10.
This year  is unusual, because the same full moon will fall after the spring equinox in both calendar systems, resulting in coinciding dates for the Easter observation.
—Pier Press seeks to facilitate informed conversation at the intersection of science and spirit by promoting Biblical and scientific literacy. This article is excerpted from one that appeared in the April 7, 2017 issue of Observations: A Pier Press® Newsletter. Subscribe today to have future newsletter issues delivered directly to your inbox.