Easter ... when is it??
Motivated by the birthday of a daughter

Okay, so today is Easter, 2008, and it's very early (March 23, 2008) and I ran across a really neato algorithm for calculating when Easter falls and ...

>You mean ... what day?
Yes. In 1935, for example, when was Easter? Or in the year 500, when was Easter? Or the year 3000, on what day will Easter fall? And in the year ...

>Yeah, okay, I get it. And you can calculate those days?
Yes, using this neat algorithm I found on the Internet.

After writing an Excel macro that executes that code, my daughter asked when Easter fell on her birthday: Apr 9.
So I included that feature. Anyway, the spreadsheet looks like this:

You can type in a year (example: 1234, in cell B4) and click a button to get Easter DAY, in that YEAR. (Example: Apr 23.)
You can also pick a day (example: Apr 9, in cell B12) and click a button to get the YEAR when Easter Sunday falls on that DAY.

>There's just a single year?
When you click the second button? No, there are probably many years when Easter Sunday falls on, say Apr 9.
However, you pick a range of years (cells D16 and D17) and the macro will run through all those years, stopping at the first year where Easter matches the DAY you picked.
For example, Easter fell on Apr 9 in the year 551.

>And you're sure of that?
Uh ... I wasn't around at the time.

Did I mention that you click on the picture to download the spreadsheet?

Click here to read a fascinating article on:

  • The Julian and Gregorian calendars (the former introduced by Julius Caesar in 45 BC).
  • When countries changed from one to t'other (Newfoundland on 2 Sep 1752).
  • Dates as understood by Orthodox churches.
  • When "leap year" occurs (Feb 24!?).
  • How the months got their name (December from decem, because it's the 10th month !?).
  • When was Christ born? (Before 4 BC !?).
  • How one counts years (there's 1 BC followed by 1 AD, but 0 is missing !?).
In particular:
  • When is Easter !

>I know! It's the first Sunday after the full moon after the vernal equinox.
After the "official'' full moon or the real full moon
... and after the "official'' vernal equinox (March 21) or the real one (when the sun crosses the "celestial" equator, which may differ by a day or two)
... or what?

Okay, what date is 02-03-04?
Is it March 2, 2004 or February 3, 2004 or is it March 4, 2002 ... or what?