As you know, Easter Sunday moves around on the calendar. I knew it had something to do with astronomy, so I did some research to try and figure out the how and the why. My findings were complicated, but here is my best shot at it. Easter is defined as a “moveable feast”, meaning it is not a fixed to a set calendar date.
The early Christians chose to link the date of Easter to the Hebrew calendar, which is partly based on the phases of the moon. The New Testament states that the Resurrection took place on the first day of the week following Passover. Sunday is the first day of a Jewish week, and the Passover falls on the day of the first full moon after the spring equinox, which can fall on either March 20th or 21st. The result was that different churches ended up celebrating Easter on different days, and so to reduce the confusion the Roman Emperor Constantine the First organized a major summit meeting in 325 A.D. to standardize the date of Easter.
From this meeting it was decreed that Easter would be celebrated on the Sunday following the Paschal Full Moon. Paschal is from the Greek word “pascha” and the Hebrew word “pesach”, which means “Passover”. Astronomers approximated astronomical full moon dates for the Christian church, calling them Ecclesiastical Full Moon dates because they were based on a lunar calendar. From 326 A.D. forward the Paschal Full Moon date has always been the first Ecclesiastical Full Moon date after March 20 (which was the spring equinox date in 325 A.D). Okay, I admit that’s still pretty fuzzy. The rule of thumb is that Easter occurs on the first Sunday after the first full moon after the spring equinox, which works pretty well, so you could leave it at that.
In my research I stumbled across some other interesting tidbits about Easter time. The Wednesday before Easter is known in some religious circles as Spy Wednesday, the day that Judas Iscariot first conspired with the Sanhedrin to betray Jesus. The Thursday before Easter is called Maundy Thursday, from the Latin word “mandatum”, meaning commandment. This date is a commemoration of the commands Jesus gave at the Last Supper, and was celebrated through a ceremony where people of authority washed the feet of the poor, and by giving alms in memory of Christ’s actions on that day. The Saturday before Easter is called “Great” or “Holy Saturday”, commemorating when Jesus physically “rested” in the tomb.