A couple times last week the question as to why the date of Easter was movable was brought up. My cousin asked in an email:

Why is calculation of Easter Sunday based on generic/basic astronomy? The first Sunday after the first full moon after the spring equinox?

A few days later another friend remarked during dinner that she thought it was strange that the date for Easter was always different. “Why don’t they just celebrate it on the same day every year,” she said.

I realized that this might not be common knowledge for most people so I thought I would shed a little light on the topic, and sum up a very complicated story that’s led to today’s calculation of Easter.

Easter & Passover

So first you have to know that Easter is theologically linked with Passover, and in fact the Last Supper is believed to have been a Seder Meal. So in the Early Church they would celebrate Easter when the Jews celebrated Passover.

The traditional term for the feast of the Resurrection is not Easter (a word which is believed to refer to a German Goddess), but in latin is Pascha (Pascua in Spanish, Pasqua in Italian, etc…), a word that comes from the Greek Πάσχα, which is derived from the Hebrew פֶּסַח which means Passover. In Armenian the original term is Բասեք or Pasek (meaning Passover), but has become Զատիկ or Zadik.

Some background: according to the Old Testament, Passover is “the fourteenth day of the first month” (Lev 23:5). Ancient Israel would determine the first month based on the Vernal Equinox, as a lot of eastern peoples did and do (Persian New Year or Nowruz happens in March for that reason). So they would calculate out when the equinox and when the next full moon was, and that was Passover.

So, back to Easter. Early Christians would go and knock on their neighborhood Rabbi’s door and ask when Passover was going to be, and that’s when they would plan their Easter celebrations. This was partially done because it was believed that Christians lacked the necessary “instruction” to figure it out for themselves. This led to some issues.

By the 2^nd century, due to miscalculation some Jewish communities were fixing Easter on dates that occurred before the Vernal Equinox, like in the first week of March. This didn’t correspond with the Bible, so that was a problem. Also, some Jewish communities, due to their distant proximity from one another and lack of communication, would set Passover on different days in the year, basically meaning that it would be celebrated twice in the same year.

The other theologically relevant issue was that many people felt that Easter should be always celebrated on Sunday (this isn’t the case with Passover, obviously), because of its role in creation and its significance as the first/last day of the week, and because it is traditionally accepted that the Resurrection occurred on Sunday.

A Christian Computation

Under the circumstances the Roman Church began coming up with its own computations, and settled on the one we use today. First Sunday after the first ecclesiastical full moon which occurs on or after the Vernal Equinox. In the beginning this was only used in the West. Churches in the East kept the old custom. This became a hot button issue by the 3^rd Century, as many felt that celebrating Easter on different days in the Christian Church violated its oneness and catholicity (universalness).

Why a full moon?

The real answer regarding the question as two why the date of Easter is changes, is this Ecclesiastical Full Moon business. Easter computations use a Lunisolar Calendar which differs from the Solar Calendars we use today. Calculating the date of Easter is actually fairly complicated, and I’m not prepared to parse it out myself so I’ll quote Wikipedia.

In each solar year (January 1 to December 31), the lunar month beginning with an ecclesiastical new moon falling in the 29-day period from March 8 to April 5 inclusive is designated as the Paschal lunar month for that year. Easter is the 3rd Sunday in the Paschal lunar month or, in other words, the Sunday after the Paschal lunar month’s 14th day.

Its pretty complex, but in the end you can see how this relates to the original mandate for Passover’s date.

Consensus

So, by the first Council of Nicaea, under Emperor St. Constantine, all the bishops in attendance accepted the Western computation.

Its also worth noting that not a small part of this issue was related with the feeling of some, especially Constantine, that it was “unworthy thing that in the celebration of this most holy feast we should follow the practice of the Jews.” There goes that Constantine again. In fact the computation was designed so that Easter could never fall on the same day as Passover again.

Further Reading

That’s the summary. Other highlights include the attempted excommunication of the entire Eastern Church by Pope Victor in about 190 AD, Quartodecimanism, and Gregorian vs. Julian calendars.