Easter falls on slightly different days every year, and due to an astronomical error which can be traced back over a thousand years, the date which we celebrate on can be as much as a month off.
Easter is usually celebrated on the Sunday following a full moon that is either on or after the spring equinox; a rather complex astronomical variable. When the date was set back in 325 CE, they didn’t have a good enough understanding of the motion of the solar system to accurately measure these phenomena, so they estimated them instead.
Robert Cockcroft, a McMaster University astronomer and physicist, explains that the spring equinox is the point in the year when day and night are equally long as the sun passes the Earth’s equator, making its way back to the northern hemisphere and ushering in the spring. This date changes a little, falling somewhere between March 19 and 21. The church didn’t realize this and set the date to the 21st. They also determined that a full moon would be on the 14th day of the lunar month, which also isn’t necessarily true.
How did bunnies and eggs became associated with a religious holiday, anyway?
Because of these two estimations, our date for Easter is sometimes off compared to the astronomical date. This year, it all works out and Easter Sunday actually is Easter Sunday. But if the two dates fall out of sync, the consequences can be pretty dramatic. In six years, the church's date for Easter falls almost an entire month after the astronomical date.
Of course, there are also discrepancies depending on where you celebrate. Eastern Europe uses the Julian calendar, which is offset 13 days from the Gregorian calendar used in the west. This means eastern Christians won’t be celebrating Easter until May 5 this year.
Cockcroft explains that this kind of error isn’t actually uncommon in science and happens whenever our understanding for something dramatically changes. He points to the labeling of stars as an example. The stars are ordered in a non-alphabetical order, because we have discovered that the original order, by heat, is inaccurate.