Did you know that in the year 2531, Easter will be on April 8th? I’ve often wondered where in the world the date for Easter comes from and have not taken the time or exerted the effort to answer the question to my satisfaction. That is, until today.

According to Wikipedia,

Computus (Latin for computation) is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age.

The canonical rule is that Easter Sunday is the first Sunday after the 14th day of the lunar month (the nominal full moon) that falls on or after 21 March (nominally the day of the vernal equinox). For determining the feast, Christian churches settled on a method to define a reckoned “ecclesiastic” Moon, rather than observations of the true Moon as the Jews did.

There’s much more history there and varying methods of Computus, but I was pretty impressed with the version known as O’Beirne’s algorithm (HT: Mathematics Weblog)

The following process gives the date of Easter Sunday as the [tex]p[/tex]-th day of the [tex]n[/tex]-th month in year [tex]x[/tex]. It also gives the Golden Number [tex]a+1[/tex] and the epact ([tex]23-h[/tex] or [tex]53-h[/tex] whichever is between 1 and 30 inclusive). All you have to do is start with the year [tex]x[/tex] and perform 10 division operations noting the quotients and remainders.

That’s fairly easy to stick into an Excel Spreadsheet or write a quick Javascript to handle it. I’m going to point you to one I like by Steve at Mathematics Weblog – Click here

If you are interested in more calendar calculators, check out Calendrica