Priests are often asked about the selection of the date of Easter, but this year someone went a step further and asked why the date of Easter follows the lunar calendar rather than the solar calendar. This is a very good question indeed and a fairly complicated one, though a simple answer might be that the Church has always followed the Lunar calendar for the date of Easter.
In the first few centuries of the Church Easter was celebrated the day after the first full moon after the Spring Equinox, the fourteenth day of the month of Nisan, which is close to the celebration of Passover, following the lunar calendar of Judaism. Naturally, the fourteenth day of Nisan usually fell on a weekday.
Saint Augustine once remarked about the symbolism of celebrating the holy night of Easter during the full moon because it was the night when light was never banished.
This particularity of Easter falling most often on a weekday began to bother some who, having read their Scripture through and through, knew that the tomb in which Jesus was laid was found empty “on the first day of the week” (Matthew 28:1; Mark 16:2; Luke 24:1; John 20:1). The first day of the week is, of course, the day we know now as Sunday.
With this in mind, some Christians began to suggest – and even to argue vigorously – that Easter should always be celebrated on a Sunday.
A controversy of no small importance arose within the Church between the Quartodecimans (from the Latin for “fourteenth”) and those who opted for the Sunday celebration, which was addressed in 325 by the Council of Nicaea because various local Churches were celebrating Easter at varying times.
The Council Fathers decreed that the date of Easter would henceforth be on the Sunday after the Paschal Full Moon (which is, in effect, the first full moon following the Spring Equinox).
Even so, as one might expect, this decision of the Council still did not quite settle the debate and Easter continued to be celebrated on the fourteenth day of Nisan in Britain and Ireland until the year 664.
If you would like to read more on this issue, you might have a look at this very thorough discussion of the issue. The Catholic Encyclopedia of 1909 also has a thorough and helpful article by Herbert Thurston, S.J. on the so-called “Easter controversy.” (Thurston’s mind is deeply thorough, precise and orderly. I’ve read a number of his books and highly recommend him. If you happen to have one of his books lying around and no longer want, I’d love to offer a home on my shelves for it.)