Our story begins at the Last Supper, which happened on a Thursday.
It was also a Passover dinner, which means it took place under “the first full moon of Spring,” as defined by the Jewish calendar. The next day Jesus was crucified (Good Friday) and he arose on Sunday (Easter).
In the centuries that followed, different Christian groups celebrated Easter on different dates. Some used whatever day happened to be three days after Passover (which falls on a different day of the week each year). Others wanted Easter to always be on a Sunday, so they celebrated on whichever Sunday fell after Passover, regardless of how many days that was.
The Romans, being control freaks, needed to make things consistent. So in 325 AD, at the Council of Nicea, they decided that Easter should fall on the Sunday after Passover. BUT, they didn’t want the Jewish calendar to get any official credit for setting the date, so the Romans did a work-around. Since Passover USUALLY happens on the first full moon after the vernal equinox, the Romans declared that Easter henceforth would fall on “the first Sunday after the first full moon on or after the vernal equinox.”
This settled things for almost a millennium. But then, in 1582, Pope Gregory XIII realized that the long-existing Julian calendar (which was lunar) was inexact, so he replaced it with the more precise Gregorian calendar (which is solar). This was a great improvement, but in order to put it into place Gregory needed to do a one-time adjustment, and in that year, October 4th was immediately followed by October 15th (the 5th through the 14th simply disappeared)! Moreover, for the purposes of Easter, the date of the equinox was set as March 21st (even though it sometimes happens late on the 20th). And so, per Gregory, Easter would thenceforth fall on “the first Sunday after the first full moon on or after March 21st.”
Trouble was, lots of folks weren’t eager to follow papal decree. Many of the Protestant countries and their colonies didn’t adopt the Gregorian calendar for centuries (including the U.S., which did so in February 1752, thereby assuring that our first President would have his birthday recognized on THREE different dates every year, but that’s another story). And other non-Catholic Christians, like those in the Eastern Orthodox Church, have never accepted the Gregorian calendar for church use, and continue to use the Julian calendar for determining their date of Easter.
And that’s why many folks celebrated Easter 2016 on March 27th, but Christians in places like the Middle East, and Russians and Greeks around the world, will celebrate it this Sunday, May 1st, a full five weeks later.
Folks have tried on numerous occasions to rectify the two calendars, to no avail.
So in the meantime, adherents of the most prominent religion on the planet will continue to disagree about the date of their holiest day.
The Devil is in the details indeed.