kay, wait, I gotta post here, since I've been wondering about this very thing and what was just said confuses me...
I see how commemorating saint so-and-so on Julian date X as opposed to Gregorian date X would work, since that's pretty much (ultimately) arbitrary and up to the people commemorating them (ie, the Fathers set up the commemorations that way; we should stick to them, etc.).  But this involves actual astronomical data here; the date where the day and night period are equal--the Equinox--falls on or around March 21st, not April 3rd (or March 21st, Julian reckoning).  Why do we insist that the Equinox, which has already empirically happened by the Julian March 21st, be commemorated on a day other than the one it clearly coincided with, namely the Gregorian March 21st, which is seen as March 8th through Julian eyes?
Sigh...
Yup. This is one of the things those in favor of the New Calendar (and a new date for Pascha) point out. Or, as Fr. Alkiviadis Calivas puts it: It's not a new calendar, it's just not an entirely wrong calendar.
Anyway, here are some notes I took in his Holy Week seminar a few years back. They are unedited (and therefore fragmentary), but to the point nonetheless. One of his points is that Church history reveals that the early Fathers were actually quite concerned with getting the date astronomically correct and, thus, we are actually violating their spirit and intention by sticking to an incorrect calculation of the Equinox. The notes:
Our earliest source for the feast of Pascha itself come from the end of the 2nd century, when there was a dispute over the idea and time for the celebration of the event. Probably for decades before this, there was debate.
One strain of thought came from Asia Minor: that Jesus was crucified on the 14th of Nisan, regardless of the day of week, b/c this is the Jewish Passover. Whether it’s Monday or Friday, Sunday or Tuesday, we’re going to celebrate the Pascha of the Lord b/c that’s when it happened — and the celebration will be focused on the Cross (sacrificial element).
Rome said: Pascha should always fall on a Sunday b/c that’s when he was RAISED from the dead.
This controversy continued until the first Ecumenical Synod. The Synod took this issue up and declared: Pascha should be on Sunday. But in order to make sure there is never more than one Pascha in one year, the Synod declared that Pascha must be AFTER the first full moon of the Spring equinox. You need the Spring Equinox, and the full moon, and then the Sunday. Spring Equinox makes certain a whole year goes by, and the full moon is in keeping with the Jewish thinking. It was the decided the Patriarch of Alexandria should decide when the date should fall, b/c that’s where the scholars and scientists were.
We should respect human wisdom and science. Not uncritically, but fully.
The problem arose when the scientists of the time discovered that the Jewish calendars were faulty.
Thereafter, the West began to develop little schemas, projecting when Pascha would fall in the future. So did the Eastern Alexandrian scientists and bishops. Both schemas were faulty according to modern scientific measurements of time. By the late Middle Ages, the astronomers of the East went to the Emperor of Byzantium and said: “The calendar is off...the Spring Equinox is moving into Summer.” But the Empire was being attacked on every side, and the Emperor decided not to get involved in adjusting the calendar (which would mean dropping some days — sure to cause controversy).
While the East was the first to recognize this problem, a 16th century astronomer convinced the West to adopt a new calendar (Gregorian) that was accurate.
For a number of years the Protestants would not adopt the “Papal” calander. By the end of the 18th century, every place in Europe had adopted the more accurate calendar — except for the Orthodox lands.
In 1920s, there was a Pan-Orthodox Council in Constantinope. Some of the Orthodox Churches adopted the "new" calendar. But, since not everyone agreed, the Ecumenical Patriarch decided that *all* Churches should continue to use the "old" (or incorrect) calendar, so that we would all at least celebrate Pascha on the same day.
The only possible days for Pascha according to the Nicaean formula: March 22 - April 25, which means, according to the "new" calendar: April 4 to May 8.
Calendar issue and Paschal tables both need to be revised, for East and West.
Synoptic Gospels say that Jesus was at the Passover on Great and Holy Thursday. But that would be near impossible to be crucified during the Passover Feast. The 14th of Nissan: The Gospel of John indicates that on the year of the Lord’s Death, the Passover occurred on a Saturday. On Friday, thefore, the lambs were being slaughtered in the Temple in preperation for the celebration of the Passover on Saturday. We follow John's account, which fits theologically and historically.