A few years ago, after reviewing the dates of Easter from year to year, I noticed a pattern that would be familiar to someone who is aware of the 3 year cycle of reading. In most instances each Easter occurs 3 or 4 days earlier than the Easter of the previous cycle, for example, easter 2010 © is 4 days earlier than is was in 2007 ©. In our last A year (2008) easter was March 23, finishing a procession that began in 1984 (A) when it was April 22, Easter 2011 (A) will go back to April 24.
The date of Easter was set on the first Sunday after the first full moon after the vernal equinox at the 1st Council of Nicaea in 325. IOW, it is literally based on the phases of the moon. The pattern you are noticing is not 100% coincidental, then, but has to do with the difference between the number of days in a solar year (365) and a lunar year (twelve cycles of the moon).
This is how the date is set, as I understand it:
Easter isn’t set based on the true astronomical vernal equinox, but the equinox as reckoned as a set date on the calendar. In the Roman Church, we use the Gregorian calendar, and set the vernal equinox on March 21. Therefore, Easter can fall anywhere between March 22 and April 25 for the Roman Catholic Church.
The Eastern Orthodox Easter also falls between March 22 and April 25, but on the Julian calendar. Since the Julian date of March 22 currently falls on the Gregorian April 3, the Orthodox Easter may fall anywhere between April 4 and May 8. There are other Oriental Orthodox Churches besides the Eastern Orthodox; some use the Gregorian and some the Julian calendar.
I think there must also be some difference of opinion as to what “after the full moon” means, because sometimes Roman and Orthodox Easter are off from each other by a week. That would come about because of a difference of opinion on whether the full moon would occur in time for Easter to fall on a particular Sunday.
Anybody who has this based on first-hand documents, feel free to correct me.
Someone on another board told me that the Eastern Easter will never come before Passover. I have never seen anything official about it. This year Passover started last night and both Easters are next Sunday. Since being told this I have noticed that when the Western and Eastern Easter are a week apart that often Passover starts during “our” Easter week and the Eastern Holy Week. (I don’t remember if I looked every year but when I did Passover was in between the two Easters)
Now this makes it more confusing since Passover is based on the Jewish Lunar calendar.