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Revision as of 20:56, 19 November 2004 by Charles Matthews (talk | contribs) (cat)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)The Doomsday algorithm is a way of calculating the day of the week. It makes use of the fact that, in each year, the following dates are all on the same day of the week:
February 28 (or February 29 if it's a leap year) April 4 May 9, June 6 July 11, August 8 September 5, October 10 November 7, December 12
This day of the week is called Doomsday and is derived from the last day in February, which, depending on leap years, is the 28th or 29th. The dates listed above were chosen to be easy to remember; the ones for even months are simply 4/4, 6/6, 8/8, 10/10, and 12/12. The others (5/9, 9/5, 7/11, and 11/7) are based on the phrase "I work from 9 to 5 at the 7-11."
Therefore, if you know what day of the week Doomsday — the last day in February — is for a given year, you can easily determine the day of the week for any other date in that year, by finding the nearest Doomsday.
For dates in March, March 7 falls on Doomsday, but the pseudodate "March 0" is easier to remember, as it is necessarily the same as the last day of February.
The Doomsday algorithm was invented by John Horton Conway. It can be used for either the Gregorian Calendar or the Julian Calendar, but note that Julian calendar Doomsdays usually occur on different days from the Gregorian calendar Doomsdays.
An example
Suppose you want to know which day of the week April 17 2004 is. In the year 2004, February 29 is a Sunday. Therefore, Doomsday for 2004 is Sunday. This means that April 4 is also Sunday, so April 18 is a Sunday as well (since it's two weeks after April 4). April 17, being the day before it, must be a Saturday.