From How is the date of Easter determined? - The Straight Dope
Just a small question. Cecil says in his explanation If the full moon after the equinox falls on Sunday, Easter follows a week later.
At what point on the earth is this determined? The full moon may occur Saturday in one time zone and Sunday in another.
Just curious.
As an Episcopalian Acolyte (that’s alter boy for you HRC guys), I was always taught that there must be an intervening Friday. As in, the first Sunday after the first Friday after first full moon after the vernal equinox. So if the full moon after the equinox is on a Saturday, Easter will not be the next day, but a week later.
Friday has nothing to do with it.
In answer to the OP, it has essentially nothing to do any more with the time of the actual full moon; the whole thing is calculated according to a pre-set cycle of lunar months and an assumption that the vernal equinox is March 21, regardless of whether or not that is actually the case.
This is an obvious difficulty. It is solved, as **DSYoungEsq **notes, by basing the rules for the calculation of the date of Easter on a completely hypothetical moon and equinox, resulting in the same Easter date throughout all time zones.
Must be the hypothetical moon we sent the hypothetical moon landings to. 
Theoretically, Easter is the Sunday after (never on) Passover, although some early Christians celebrated Easter on Passover. However, there are five different calendars here, with different hypothetical moons and hypothetical equinoxes.
The Jewish calendar is a lunar calendar of 12/13 months a year.
After the earliest days, most Christians were gentiles, and wanted to use instead the Julian Calendar of Julius Caesar, which was in universal use in the Roman Empire, so they made some calculations for when Passover, and therefore Easter, should fall. This is still used by conservative Eastern Orthodox Christians, who believe that ancient tradition is more important than astronomy.
In 1582, Pope Gregory XIII redid the calendar, because it had become obvious that Julius Caesar’s figure of exactly 365.25 days per year was a little too much. Ten days in October were skipped to bring the Equinox back to the day that it had fallen on at the time of the Council of Nicea in 325, when the rules for Easter were first officially set. At the same time, the rules for the hypothetical moon were completely rewritten. This is, of course, the Gregorian Calendar.
For a long time, Protestants refused to have anything to do with it out of spite, but, after about 200 years, the Church of England decided that, no matter how evil they thought the Pope was, the astronomical facts were plain. However, to avoid copying Rome, instead of using Gregory’s new hypothetical moon, they made some patches to the old one. This is the “New Style” calendar, and, in September of 1752, eleven days were skipped. At present, the Gregorian Calendar and the New Style Calendar give the same results, but they will not always.
In the 20th century, some reform-minded Eastern Orthodox decided to tackle the problem, and came up with yet another calendar, the “Revised Julian”, which by then involved skipping thirteen days. Like the New Style Calendar, it is in sync with the Gregorian Calendar now, but will not always be. On the whole, and for whatever it’s worth, the Revised Julian is the most astronomically accurate.
Cecil left out my favorite part, the name of the calculation: Computus.
This was not the case as recently as last year: There was no intervening Friday, and Good Friday fell on March 21 itself.
March 21 isn’t always the vernal equinox, I believe.
Not astronomically, but it’s the “official” vernal equinox for purpose of Easter calculations.
Yes, that’s true, as I noted in my earlier post. But that doesn’t mean that what Backwater Under Duck said isn’t true, because it could be the case that whenever the Friday falls on Mar 21, the equinox is on the 19th or 20th in real life, and thus the maxim quoted is true in terms of the real equinox, yadda yadda. I’m not saying I think it IS true, just that to prove it wrong, you need to find a Friday that is not “between” the real equinox and the Easter that follows.
What adds to the complication is that many other Christian festivals are dated by their relationship to Easter. These are known as moveable feasts (I’ve always loved that term, which uses an earlier sense of feast meaning festival, they’re both from the same Latin root).
These include Quasimodo Sunday (the first Sunday after Easter, for years I never realized that Hugo didn’t just invent the name of the hunchback), Shrove Tuesday, Palm Sunday, etc.
It seems to be up in the air as to whether Easter itself is a moveable feast, a little like Russell’s ‘The barber shaves all those who don’t shave themselves. Who shaves the barber?’, I guess.
Pray tell, what exactly is the difference among the three calendars that will cause them to become out of sync? Do they not all skip the same leap days, or what?
Powers &8^]
Much too complicated to answer here. The Wikipedia article s.v. Computus will give you an introduction to the subject.
Cool. Ididn’t either until just now.
Named for the opening words of the Introit (i.e. entrance chant) of that Sunday’s mass, and a Mode VI chant that we always enjoy singing each year:
This is fascinating! There was some talk a few years ago about the Catholic Church possibly fixing the date of Easter as a way of avoiding the whole mess and (I think) as a step to making the Eastern and Western ecclesiastical calenders easier to reconcile. (Presumably the Orthodox would have to do the same thing.)
I remember wondering what the Baptists and the fundamentalists would do if that happened. They celebrate Easter, but I doubt any of them know how to calculate it - I suspect they just celebrate it when the calender makers tell them to. If the RCC changed, the mainline churches would go along to be ecumenical, but the Baptists and fundamentalists would be loath to admit the Catholics have the authority to do anything. I figured they would quickly republish lots of the long-established tables that go through 2199 or something, and pretend that they were an ancient tradition of their own handed down straight from Jesus. But what would they do after that?
But if the Catholic and Anglican calendars diverge at some point, that causes similar problems, even if the Catholics keep Easter as it is. The Baptists will have to grant authority to one of the two churches and admit they were just going along with the Papists (or the English) all this time, or else come up with their own Computus and admit they didn’t really have a method before! (I suspect the fundies would gradually decide that Easter was unbiblical after all.)
Because the Orthodox and Western calendars differ(Julian and Gregorian) that’s why the date of fixed religious celebrations will not coincide.
But because of the rules for determining the date of Easter sometimes the Eastern and Western churches will differ by as much as a month, and sometimes Easter will be celebrated on the same day. But as time goes by, and the Julian and Gregorian dates continue to diverge, the cases of same day Easter will become less common, until finally it will not happen at all.
Missed the edit window. The following link is to a series of tables of the dates Easter will fall on up to the year 3000. And as the intro to these charts says, that’s assuming our calendars don’t change, nor does the method of determining the date of Easter. It notes that after 2698 that Orthodox and Western Christians will no longer have common dates for Easter.
In the Gregorian Calendar and the Revised Julian Calendar, years evenly divisible by four are leap years, except that years evenly divisible by 100 are not leap years. In the Gregorian Calendar, it continues, “unless they are divisible by 400, then they are leap years.” In the Revised Julian Calendar, it goes, “unless they leave a remainder of 200 or 600 when divided by 900, then they are leap years.” Thus, the calendars will diverge by a day in 2800.
The British New Style Calendar is the Gregorian Calendar – there is no difference. The “divergence” John W. Kennedy refers to is solely liturgical, and has to do with the lunar calendars and corrections that are used to calculate the date of Easter, because just looking at the damn MOON is somehow unsound.