Good morning, HappyEaster, and welcome to the latest Easter Sunday in living memory. It might not be the latest Easter you've ever experienced, if you're over retirement age that is, because Easter fell one day later back in 1943. And it won't be the latest Easter you'll ever experience, assuming you live for another quarter of a century, because it'll fall one day later in 2038 too. But for a sizeable middle-aged proportion of the world's population, Easter on 24th April is as late as it'll ever get.
And here, very simply, is why it's so late. Easter is always the first Sunday after the first full moon after the spring equinox. And this year, 2011, everything in that list has happened really really slowly...
Equinox*: Monday 21st March Full Moon**: Sunday 17th April Easter Sunday: Sunday 24th April
There was a full moon on Saturday 19th March (the super full moon, you remember). But 19th March was slightly too early to trigger the date of Easter because the spring equinox hadn't quite taken place. So things skipped ahead 29 days to the next full moon, which was last Sunday 17th April. But the rules state that Easter has to be the next Sunday after the full moon, so here we are on Easter Sunday 24th April.
* Sorry to get complicated, but this year the true spring equinox fell just before midnight GMT on Sunday 20th March. But the church chooses to ignore such astronomical fripperies, because Greenwich never once appears in the Bible, and so takes the date of the equinox as 21st March whatever. Don't worry, be it Sunday 20th or Monday 21st, Easter 2011 would still have ended up being today. ** Sorry to get more complicated, but this year the true Easter full moon occurred around 3am GMT on Monday 18th April. But the church chooses to ignore such astronomical fripperies, because Greenwich never once appears in the Bible, and so takes the date of a "Paschal full moon" on 17th April instead. Don't worry, be it Sunday 17th or Monday 18th, Easter 2011 would still have ended up being today.
The only way to get a later Easter than 24th April is to have full moons on 20th March and 18th April. This happens roughly every 19 years, which is a fairly common event. But we only get the ultimate late Easter when 18th April is a Sunday, delaying Easter until Sunday 25th April. There are seven possible days of the week on which 18th April can fall, and only when it's a Sunday does the ultimate late Easter occur. A one in 19 chance, multiplied by a 1 in 7 chance, that's an event of 1 in 133 year rarity.
Here's a list of really late Easters, so you can see how rare they really are: Easter on 21st April: last happened 1957, next happens 2019 Easter on 22nd April: last happened 1984, next happens 2057 Easter on 23rd April: last happened 2000, next happens 2079 Easter on 24th April: last happened 1859, next happens 2095 (and this year, of course) Easter on 25th April: last happened 1943, next happens 2038 (but then not again until 2190)
Easter trivia Here are two excellently long lists of Easter dates: 1875 to 2124; 1583 to 3000 The cycle of Easter dates repeats every 5,700,000 years. During this period, Easter falls on 24th April only 81,225 times (less than 1½% of the total). Although Easter can fall on any date from 22nd March to 25th April, the two most likely dates are 19th April and 18th April. Almost exactly half of our Easters occur on the same date 11 years later (but never more than four in a row). For example, Easter will fall on 21st April in 2019, 2030, 2041 and 2052, but not in 2063. Consecutive Easters are always separated by 350, 357, 378, or 385 days (that's 50, 51, 54, or 55 weeks). For example, Easter 2011 is 385 days after Easter 2010, but 350 days before Easter 2012. So we've just had the longest possible gap between Easters, and we're about to enter the shortest. Until this weekend, the UK's warmest Easter was in 1984, when the temperature in Jersey on 22nd April reached 26°C. Tons more Easter weather records can be found here. It's extremely unlikely to have an extremely early Easter (2008) so close to an extremely late Easter (2011), and this won't happen again until 2160/2163.