Why isn’t Easter celebrated on the same date every year?

It had all the makings of a spirited religious debate: More than 300 bishops representing different factions of the Christian church met during the 4th century in what is now Turkey.

One of the subjects on the table was Easter.

Early Christians disagreed about whether Easter, the faith’s most theologically significant holiday, should be celebrated in conjunction with the Jewish holiday of Passover. Syrian churches, which wanted the holidays to fall on the same day, were in the minority.

Bishops meeting at the Council of Nicaea in 325 A.D. tried to resolve the disagreement. All churches would determine the date of Easter using the method of Christians in Rome and Alexandria, Egypt, the bishops wrote to the Alexandria churches after the meeting.

The letter, however, left a key question unresolved.

“It does not tell you how to make the calculation,” said Susan Wessel, a theology and religious studies professor at Catholic University.

Pope Francis celebrates Palm Sunday Mass in St. Peter’s Square at the Vatican as the Catholic Church enters Holy Week. (Gregorio Borgia/AP)

The eventual result was another divide among Christians. Catholics and Protestants now celebrate Easter on the first Sunday after the first full moon following the spring equinox, placing the holiday between March 22 and April 25.

While most churches calculate the date with the Gregorian calendar created by Pope Gregory XIII, Orthodox churches use the older Julian calendar established by Julius Caesar. Orthodox Christians celebrate Easter between April 4 and May 8.

As a result, anyone trying to plan an egg hunt or calculate when stores will stock chocolate bunnies has to take a fresh look at the calendar each spring to find the right dates. Easter falls late this year, with most churches celebrating it on April 21 and Orthodox Christians recognizing it a week later.

The bishops’ original disagreement over when to recognize the holiday centered on how directly to link Easter to Passover, when Jews celebrate their liberation from slavery in ancient Egypt. In the biblical story, God inflicted plagues on the Egyptians. Jews who marked their doors with lamb’s blood were passed over and did not suffer the plague of death of their firstborn son.

Early Christians in Syria, who lived in proximity to Jews, wanted to maintain the theological link between Passover and Easter, Wessel said. They drew a parallel between the sacrificial lambs of Passover and the figurative lamb of Jesus Christ, who they believed died on the cross to atone for humanity’s sins.

Roman and Alexandrian Christians, meanwhile, thought Easter should be on a Sunday because of the Christian belief that Jesus rose from the dead on that day. This group also tried to differentiate themselves from Judaism, Wessel said, and did not want to link Easter directly with Passover.

The Roman church was starting to be seen as the epicenter of the faith, and its ideas carried weight, Wessel said. That influence, combined with negotiations to make sure the Alexandria church was on its side, helped it win the debate.

Early Christians’ exact reasoning for deciding that Easter should be on the Sunday after the first full moon following the spring equinox remains somewhat elusive. The significance of the full moon relates to how Passover’s date is determined, Wessel said. The date of this “Paschal Full Moon” is calculated based on lunar cycles and can differ slightly from the actual full moon, Panos Antsaklis, an engineering professor at University of Notre Dame, wrote in a 2005 paper.

The significance of the spring equinox may be its association with rebirth, Wessel said, but there is no proof that the bishops referenced it for that reason.

Even after this method for finding the date of Easter was created, the disagreement did not fully die.

“By no means is it the case that from the Council of Nicaea onward, it’s all established and everything is the same,” Wessel said.

In the mid-5th century, she said, Pope Leo I acknowledged that Alexandria had diverged from Rome and begun calculating the date of Easter using a different calendar. The realization sparked tension, Wessel said, but the factions maintained their separate systems.

The method for computing the date is periodically challenged to this day. Bishops at the Second Vatican Council in 1963 proposed fixing Easter on a specific Sunday, according to one of the council’s constitutions. The World Council of Churches in 1997 also tried unsuccessfully to standardize the process.

“It’s a continuing debate because of the complexity of the calculations,” Wessel said. “Nobody’s really sure how each other is doing it and their reasons for doing it.”

In Wessel’s estimation, though, that’s probably fine with modern-day churches. In the grand scheme of religious debates, she said, the date of Easter is one of those topics on which Christians can agree to disagree.