Good morning, HappyEaster, and welcome to the earliest Easter Sunday that you will ever experience. And here, very simply, is why it's so early. Easter is the first Sunday after the first full moon after the spring equinox. This year, 2008, everything in this list has happened really really fast...
Equinox: Friday 21st March Full Moon: Saturday 22nd March Easter Sunday: Sunday 23rd March
Bang, bang, bang. You can't beat that.
Day of Month
As I mentioned on Friday, we have a full moon on 22nd March once every 19 years. It's a common event. The reason that 2008 is so special is that 22nd March is a Saturday, allowing 23rd March to be a Sunday. There are seven possible days of the week on which the 23rd can fall, and only when it's a Sunday does the ultimate Easter occur. A one in 19 chance, multiplied by a 1 in 7 chance, that's an event of 1 in 133 year rarity.
OK, so once you know the date of the first spring full moon in any year, how do you find the date of the next Sunday? Ah, well, that's where the Sunday Letter comes in. Know the Sunday Letter and you can work out which of the next seven days is a Sunday. Every year is assigned its own Sunday Letter (one of the letters from A to G), roughly corresponding to the seven different days of the week on which January 1st can fall. This Easter the Sunday Letter is E. Next year it's D, then C, then B, and then for 2012 (a leap year) it skips A and flips back round to G. There's a ready-made list of Sunday Letters for the 20th and 21st centuries here.
You can then use today's big long table, reproduced from the Book of Common Prayer, to work out the date of Easter. For any particular year you need to know only two things - the Golden Number (see Friday's post) and the Sunday Letter. Start in the first column with the Golden Number. Then move down the table until you find the next row with the correct Sunday Letter. That's the date of Easter Sunday. Simple?
For example, in 2008 the Golden Number is 14 (XIV), so start on the March 22nd row. The Sunday letter is E so move down to the next E, which is March 23rd. And so Easter is March 23rd. For example, last year the Golden Number was 13 (XIII), so start on the April 2nd row. The Sunday letter was G so move down to the next G, which is April 8th. And so Easter was April 8th. Oh, and if the full moon is on a Sunday, then Easter jumps to the following Sunday. That's to make sure it's never celebrated on the same day as the Jewish Passover.
Whichever date is associated with the Golden Number, Easter can fall on any of the seven dates below it. So not every year with a Golden Number of 14 has a really early Easter. In 2027, for example, Easter will fall on March 28th (Sunday Letter C) and in 1970 Easter was as late as March 29th (Sunday Letter D). Easter 2008 is only fabulously early because the Golden Number is 14 and the Sunday Letter is E.
Meanwhile down at the bottom of the table, Easter can sometimes run very late. This tends to happen when the Golden Number is 6 (as in 2000), 9 (as in 2003) or 17 (as in 2011). The latest Easter of all (April 25th) can only happen in a year with Golden Number 6 and Sunday Letter C - years such as 1943 and 2038. But we've got a close contender coming up very soon. In 2011 everything in the Easter calculation list is going to run really really slowly...
Equinox: Monday 21st March Full Moon: Sunday 17th April Easter Sunday: Sunday 24th April
So, calendar freaks, do come back in three years time. In 2011 we've got an extremely late Easter on our hands. And you just wouldn't believe how rare it is to have an extreme late Easter so close to an extreme early Easter...