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The Doomsday rule or Doomsday algorithm is a way of calculating the day of the week of a given date. It provides a perpetual calendar because the Gregorian calendar moves in cycles of 400 years. This algorithm for mental calculation was devised by John Conway〔John Horton Conway, "Tomorrow is the Day After Doomsday", Eureka, volume 36, pages 28–31, October 1973.〕〔Richard Guy, John Horton Conway, Elwyn Berlekamp : "Winning Ways: For Your Mathematical Plays, Volume. 2: Games in Particular", pages 795–797, Academic Press, London, 1982, ISBN 0-12-091102-7.〕 after drawing inspiration from Lewis Carroll's work on a perpetual calendar algorithm.〔Lewis Carroll, "To Find the Day of the Week for Any Given Date", Nature, March 31, 1887.〕〔Martin Gardner, "The Universe in a Handkerchief: Lewis Carroll's Mathematical Recreations, Games, Puzzles, and Word Plays", pages 24–26, Springer-Verlag, 1996〕 It takes advantage of each year having a certain day of the week (the ''doomsday'') upon which certain easy-to-remember dates fall; for example, 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February all occur on the same day of the week in any given year. Applying the Doomsday algorithm involves three steps: # Determine the "anchor day" for the century. # Use the anchor day for the century to calculate the doomsday for the year. # Choose the closest date out of the ones that always fall on the doomsday (e.g. 4/4, 6/6, 8/8), and count the number of days (modulo 7) between that date and the date in question to arrive at the day of the week. This technique applies to both the Gregorian calendar A.D. and the Julian calendar, although their doomsdays will usually be different days of the week. Since this algorithm involves treating days of the week like numbers modulo 7, John Conway suggests thinking of the days of the week as "Noneday" or "Sansday" (for Sunday), "Oneday", "Twosday", "Treblesday", "Foursday", "Fiveday", and "Six-a-day". The algorithm is simple enough for anyone with basic arithmetic ability to do the calculations mentally. Conway can usually give the correct answer in under two seconds. To improve his speed, he practices his calendrical calculations on his computer, which is programmed to quiz him with random dates every time he logs on.〔Alpert, Mark. "Not Just Fun and Games", Scientific American, April, 1999.〕 == Doomsdays for some contemporary years == Doomsday for the current year in the Gregorian calendar () is . For some other contemporary years : Notes: Fill in the table horizontally, skipping one column for each leap year. This table cycles every 28 years, except in the Gregorian calendar on years multiple of 100 (like 1900 which is not a leap year) that are not multiple of 400 (like 2000 which is still a leap year). The full cycle is 28 years (1,461 weeks) in the Julian calendar, 400 years (20,871 weeks) in the Gregorian calendar. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Doomsday rule」の詳細全文を読む スポンサード リンク
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