Apsuvax.1014 net.games utzoo!decvax!ucbvax!ARPAVAX:mhtsa!allegra!psuvax!sibley Mon Dec 21 15:49:52 1981 Doomsday 'Doomsday' is a method for figuring out mentally what day of the week a given date falls on. It was designed by J.H.Conway of Cambridge University, England. I will describe how it works for dates in this century, but it can be applied to any century. First, Doomsday is a day of the week, by definition the day which the last day of February falls on. It changes from year to year, of course. In 1981 Doomsday is Saturday. In 1900 Doomsday was Wednesday. The method has two basic parts: a way to figure out what Doomsday is for any year (in this century) and a way to remember one day in each month which falls on Doomsday regardless of the year. The following table gives one date for each month which is a Doomsday. jan 31 or 32 jul 11 = 7+4 feb 28 or 29 aug 8 mar 7 = 3+4 sep 5 = 9-4 apr 4 oct 10 may 9 = 5+4 nov 7 = 11-4 jun 6 dec 12 Actually, this is designed to be easy to remember. Jan 31 (or 32 in leap years--it doesn't make any difference that Jan 32 does not exist) is easy to remember. Feb 28 (or 29 in leap years) is the definition of Doomsday. There is a simple system for the remaining months, as follows. The even numbered months have Doomsday on that number, as June, the 6th month, has Doomsday on its 6th day. For the odd numbered months, either add or subtract 4 from the number of the month--add if the month has 31 days and subtract if it has 30 (add for long odd months, subtract for short odd months). After a while you may recognize dates important to you which also fall on Doomsday, e.g., birthdays. Or, Independence Day is a Doomsday. Thus, for instance, in 1981 Dec 12 is a Doomsday = Saturday. >From this information it is easy to figure out what day of the week any day this month is--more on this later. To figure out what Doomsday is in any year (this century) do the following. Start with the last 2 digits of the year, e.g., 81 for 1981 and add up its quotient by 12, the remainder, and the number of 4's in the remainder, as in 6 dozens in 81 +9 left (81-72) +2 4's in 9 __ 17 total Add this (or this mod 7) to Wednesday (Doomsday 1900). In 1981 we have Doomsday = Wed + 3 = Sat. This works because each year pushes Doomsday ahead one day, except in leap years it's two, and "a dozen years is but a day." (For other centuries you also need a method to figure out what Doomsday is for that century. Beware of calendar system changes.) Conway originally recommended the method be applied as in this example (although he has changed his mind since then): For Nov 22, 1946 first observe that for 1946 we have 3 dozen + 10 + 2 (4's in 10) =15 so Doomsday 1946 is Wed + 1 = Thur. Now Nov 11-4 = 7 is Doomsday = Thur, so Nov 14 is a Thur, Nov 21 is a Thur, Nov 22 is a Friday. That is, start with Doomsday for your month and add or subtract 7's until you get close. It really is easy to do in your head after a little practice. Amaze your friends. ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.