Since the early Middle Ages, all Christians have used the same method for determining the date of Easter, though they arrive at a different result. Described authoritatively in The Reckoning of Time by eighth-century English scholar Bede, “The Sunday following the full moon which falls on or after the equinox will give the lawful Easter.” The equinox is observed on March 21. This straightforward method based upon an easily observable natural phenomenon survived the Schism of 1054, when the Catholic and Orthodox Churches split from each other. Still, if you have Orthodox Christian friends or visit a predominantly Orthodox country, such as Russia or Greece, in the late spring you may find yourself celebrating a second Easter.

You can thank Gregory XIII for the extra helping of Easter eggs. In 1582, the pope instituted a new calendar for all Catholic countries, and it would eventually be adopted as the civil calendar by countries throughout the world. The old Julian calendar year, created during the reign of Julius Caesar, was 11 minutes too long. That may not seem like much, but compounded over centuries, it was causing calendar days to shift in relation to natural phenomena like the change in seasons. Consequently, by the time of Gregory's papacy, the spring equinox could no longer be observed on March 21. The new Gregorian calendar brought March 21 back into alignment with the equinox by axing 13 days from the calendar for one year. To keep things in alignment going forward, the Gregorian calendar allows a centurial year, like 1900 or 2000, to be a leap year only if it is divisible by 400. The Orthodox churches continued to use the Julian calendar. Thanks to the 13-day difference between the two, the all-important date of March 21 on the Julian calendar corresponds to April 3 on the Gregorian calendar.

For the remainder of the 21st century, Easter will fall between April 4 and May 8 for Orthodox Christians, and March 22 and April 25 for Catholics. Still, Jesus only died and rose from the dead once, not twice within two separate five-week windows. In the interest of Christian unity, some religious reformers have pushed for creating a single Easter that all churches can observe. In 1997, the World Council of Churches proposed that Easter Sunday should be determined by when the astronomical equinox is observed in Jerusalem. This solution would sidestep the issue of any church having to abandon their calendar. However, Orthodox Christians would be more dramatically affected by this change than Catholics, since the astronomical equinox is much closer to March 21 on the Gregorian calendar than it is to March 21 on the Julian calendar.

Since the last call for reform at a 2009 meeting in Ukraine, attempts to standardize the date of Easter have stalled. In the meantime, Catholics and Orthodox Christians can look forward to April 16, 2017, when their divergent calendars and calculations will lead them to celebrate Easter on the same Sunday.