This column first ran in The Tablet in January 2016
The archbishop of Canterbury Justin Welby has recently  announced discussions to redefine the date of Easter. Pope Francis and various leaders of Eastern churches have also expressed interest in a common date that all churches would celebrate together.
Easter was originally the Sunday following Passover, the first full moon of the Hebrew year. But the start of the Hebrew year varied from year to year. Jewish months, 29 days long, mirror the phases of the moon, and so every three or four years an extra month is needed to keep that lunar calendar in phase with the seasons. After the destruction of the Temple in AD 70, there was no central Jewish authority to determine when to add that month. Instead, Jews of the Diaspora relied on a Greek formula (devised in 432 BC by Meton) to add seven intercalary months over a repeating 19 year cycle. It was clever; and it almost worked.
The Roman world used Julius Caesar’s season-based calendar of 12 fixed months with one leap year every four years. It too was clever, and almost worked. A year is defined by the length of time from season to season — originally, spring to spring. Spring’s beginning is marked by a particular position of Earth’s orbit. By a nice coincidence, it takes almost exactly 365 1/4 days to return to that position. By adding one extra day every fourth year, you can keep a calendar of days in sync with a calendar of seasons. Almost.
The Council of Nicea in AD 325 assumed that the beginning of spring occurred on March 21 by Roman calendar. It defined Easter as the first Sunday following the first full moon after that date. There were two problems with that system, however.
The “almost works” nature of the formula meant that its tiny error built up over the centuries. The leap year is a bit too long; after 133 years, you’ve added one day too many. That was the logic behind the Gregorian calendar reform in 1582: after dropping the extra days accumulated since Roman times, a new rule was added eliminating leap year for three of every four “century” years. (1700, 1800 and 1900 were not leap years; 2000 was.) Of course, even with this new calendar, spring is not always starting exactly on March 21.
And by then, another problem with the Nicean system revealed itself. With Christians now living all around the world, what happens when a given full moon falls on a Monday in one hemisphere while it’s still Sunday further west?
So instead, the Gregorian calendar substituted a mathematical formula that simply attempted to imitate the Nicean system but which, for the sake of simplicity, gave up any attempt to be completely in line with the actual position of the seasons and the moon. In fact, the Gregorian Easter disagrees with a more astronomically calculated Easter nine times in this century alone.
Even in 1582, people thought about adopting a fixed date much like the current proposals suggest. But the reformers felt that keeping Easter as a moveable feast was closer in spirit to the Council of Nicea, from those happier days of Christian unity. They hoped that by keeping this spirit, the Protestant and Eastern churches would go along with the reform. In fact, however, Protestant countries resisted the Gregorian calendar for nearly 200 years; and the Orthodox never did accept it. Let’s hope any new system has better luck.
[I haven't heard anything new about this effort since the 2016 announcement...]