Last Sunday was Easter. This year was a special year, in that the Eastern and Western churches celebrated it on the same date. Easter is a variable-date feast for both families of churches, but the range of possible dates under each calendar partially overlaps. You might wonder how that difference between Rome and Constantinople came about. You may also wonder why its date is so varied, instead of being fixed like Christmas. The stories are related, and quite interesting.

It all starts with the Jewish festival of Passover, the annual commemoration of the Exodus from Egypt. In the Hebrew calendar, it falls on the fourteenth day of the month Nisan. (The preparation is on the 14th; the new day starts at evening time, so the feast itself is on the 15th.) It is a lunar calendar, with each month starting with the new moon. But purely lunar calendars have a problem. A cycle of twelve moons is shorter than a solar year. On the same day of the same month of a different year, the stars will be in a different place. Even worse, if the calendar is not corrected, the place of the seasons on the calendar drifts noticeably. The major feasts of the Hebrew calendar, however, are meant to correlate with the seasons. You want to offer the firstfruits of the harvest at harvest time. So the month of Nisan was pegged as following the ripening of the barley crop. If the month of Adar ended before the barley was ripe, you just have the month of Adar a second time. The middle of the month corresponded with the full moon, so in practice Passover fell on a full moon following the vernal equinox.

The Babylonian Exile, and the Diaspora in general, put a wrench in that system. You can’t easily track the Judean growing season from Babylon. Several methods were devised for keeping track of the right date to observe Passover in obedience to the Law. The head of the Sanhedrin did not decree a definitive standard mathematical method for calculating the Hebrew calendar until the 4th century A.D. Even then, it took some time for that method to be universally received by the Jewish community, such that Jews in Rome, Jerusalem, and Babylon all celebrated the feast on the same day. Before that time, in a given year different local communities might celebrate Passover on very different dates on the Roman Julian Calendar.

Christians began to commemorate Christ’s resurrection from a very early time. We see in the Acts of the Apostles that early Christians began gathering on Sunday, the day of the week on which Christ arose, very early on indeed. It is not clear when Christians started observing an annual feast of the Resurrection,* but the practice was already underway in the 2nd century. Two main forms of observance emerged: celebrating the Resurrection on 14 Nisan (first day of Passover), and celebrating it on the Sunday after that date. The Fourteenthers (aka Quartodecimans) were mainly based in Asia Minor, while the “Sunday after Passover” practice was observed elsewhere, most notably in Rome. There was fierce controversy in Rome on the matter in the later 2ndcentury. Remember, Rome as capital housed people from all over the Empire, so it was a natural place for the two practices to bump elbows. When the families from Asia Minor start feasting several days early, it raises a ruckus. For the most part, the parties of both sides agreed it was not worthy of schism. (Victor, bishop of Rome between 189 and 199, was a notable exception.) When the dispute came to a vote at regional councils (including one in Palestine), the “Sunday after Passover” practice won out. The Fourteenthers gradually disappeared, and the label slowly became a handy slur to hurl at the other side in a debate over Easter dates. (The “fighting words” use of the label makes it hard to determine exactly when the faction died out.)

That whole time, most Christians used a simple method to find out the date of Passover on the civil (Julian) calendar: ask a Jewish friend. By the early fourth century, a lot of Christians were unsatisfied with that practice. The distance between the Christian and Jewish communities had gradually grown, and leaders on both sides were increasingly concerned with marking and fencing the border lines. Also, a lot of Christians noticed the wide local variation in calendar calculation. They wanted to be able to calculate their feast on their own, and there was no universally-accepted Jewish system of calculation to copy. Other Christians were quite attached to the “ask a rabbi” method.

Their disagreement came to a head at the Council of Nicaea in 325. (Yes, Constantine may have called it to settle the dispute over Arius, but the various churches had several of their own agenda items that did not concern him.) At the council they agreed that Christians needed to draw up their own system to determine the date. They agreed that Easter should be observed on the first Sunday following the first full moon after the vernal equinox. That roughly paralleled their understanding of how the Jewish calendar customs worked out on an astronomical model. The council also tasked the bishop of Alexandria, a wealthy church in a city full of observatories and astronomy buffs, to keep track of it for everyone. Thus began the practice of the bishops of Alexandria sending an annual letter to all the churches they recognized as orthodox, telling them the year’s Easter date.

As anyone who has worked on a committee knows, an agreement made at a meeting does not always translate into everybody following the same plan. It took decades to get everybody on board, to iron out a standard date for the equinox, tables of standard full moon dates, and the resulting table of standard Easter dates. For a while, more than one set of Easter tables were used. (Notably, the Irish churches used a different Easter table than the churches in Gaul and Rome through the seventh century.) Eventually, by the 8thor 9thcentury, pretty much all but the most remote, difficult to contact churches were using basically the same tables. (In church history, any important debate takes about a century to settle. Less important ones can go on longer.)

So why the modern difference between East and West? Blame Copernicus. The astronomical revolution of the 16thand 17thcentury allowed for precise calendar calculations. Until the atomic clock, careful use of astronomy was the best way to keep track of dates, months, and years. Pope Gregory XIII commissioned a team of astronomers to reform the old Julian calendar. Set in ancient Rome, its system of leap years pulled the calendar 3 days off the solar cycle every four centuries. That meant the vernal equinox was ten days off from when it was (traditionally) set at Nicaea. That made it harder for people in distant regions to calculate the date of Easter. So Gregory had his experts reform the leap year system, reclaim the ten missing days, create a better table of the full-moon cycle, and thus reform the Easter tables. He decreed the new calendar in 1582.

Most Catholic countries adopted the new calendar quickly. Most Protestant territories initially rejected it on political and religious (read: anti-papal) grounds. But slowly the astronomical arguments for it won out. England held out until 1752. Eastern Orthodox Russia did not use it as the civil calendar until the Bolsheviks took power, and Greece waited until 1923. At that time, some Eastern Orthodox churches adopted a revised Julian Calendar that is mostly identical to the Gregorian, but kept the old Easter tables. (Not all of them accepted it, which is why there are breakaway “Old Calendar” groups, and not all Orthodox jurisdictions in the U.S. observe Christmas at the same time.) Particularly, their new calendar kept the Julian (Eastern Orthodox) date for the equinox, which is now fourteen days off the Gregorian (Western churches) date. The two systems mark their calendars off different full moons on most years.

Maybe one day we’ll settle that difference and all observe together. Until either Christ returns, or someone else decides to adjust the tables again. Whichever happens first.

In sum, the Christian Easter(s), and the liturgical year observances that are set relative to it, come from an attempt to create a Christian calendar to match the Jewish Calendar. A Christian Nisan, so to speak. And as was the case within the Jewish community, it took a lot of time, ink, and flared tempers to work it out.

*Let’s dispatch one misconception. The fact that we call the holiday “Easter” does not reveal some hidden pagan roots to the holiday. It certainly has nothing to do with the goddess Ishtar. English and German use names based on “east”, as a reference to the rising sun. In fact, words for “east” tend to come from words for “rising.” It makes a kind of sense, as Jesus Christ is the Sun of Righteousness, risen with healing in His wings. (Malachi 4:2) But most other languages call the feast day by some variant of “Pascha,” from the Hebrew pesach (“pass-over”). Instead of using a loanword or a calque like “Passover,” old-time German and English speakers created their own punning word for it, and that word stuck. Centuries later, it became a stumbling block for preachers and pamphleteers with too keen an eye for seeing ancient pagan roots in traditional Christian practices. That is the common farce that humans are prone to, a product of poor memory, excess cleverness, and axes to grind.