For most holidays we celebrate, there’s a fixed date to mark the occasion. New Year’s Day, obviously, is always January 1st. You might find yourself in the doghouse if you don’t remember a Valentine’s Day gift for your significant other every February 14th. St. Patrick’s Day is always March 17th. The list goes on.
Some holidays fall on certain days of certain months, but don’t depend on a specific date. President’s Day is always the third Monday in February. Memorial Day always falls on the final Monday of May. Labor Day is always the first Monday in September.
Then there are days like Easter, whose date can vary by more than a full month, anywhere between March 22nd and April 25th. I could have sworn that it has been as late as May, but a look at a list of dates on which it has fallen in the past disproves that errant belief. Still, with more than a month of variance possible from one year to another, one has to ask: what’s the deal with Easter?
The easy — and not entirely correct — answer is that Easter occurs on the first Sunday after the first full moon following the vernal equinox. The truth is that this only really worked through the year 325 A.D. That’s when the Council of Nicea decided to standardize things, coming up with the Paschal Full Moon based on astronomical calculations. The Paschal Full Moon’s date determines the date of Passover. Therefore, Easter is guaranteed to fall after Passover.
The longer answer, therefore, is that Easter falls on the first Sunday following the Paschal Full Moon. The Paschal Full Moon, for what it’s worth, isn’t necessarily the same date as the lunar full moon. (It can vary by two days or so one way or the other.) But because we use the Paschal date, we can determine the dates for Easter going forward without having to check a lunar chart. That’s how we know, for example, that in 2014, the Paschal Full Moon (and thereby, Passover) occurs on April 15th, and that Easter Sunday is April 20, 2014.
That’s probably more than you ever wanted to know, but you can always safely tell someone who asks why Easter’s date varies year to year that Easter is always the first Sunday after Passover. That much is always true, no matter how far back you go in the history of Easter celebrations.