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From: [email protected] (Alex Lopez-Ortiz)
Newsgroups: sci.math,news.answers,sci.answers
Subject: sci.math FAQ: dayofweek
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Date: 17 Feb 2000 22:55:53 GMT
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Archive-name: sci-math-faq/dayofweek
Last-modified: February 20, 1998
Version: 7.5


     How to determine the day of the week, given the month, day and year

  First a brief explanation: In the Gregorian Calendar, over a period of
  four hundred years, there are 97 leap years and 303 normal years. Each
  normal year, the day of January 1 advances by one; for each leap year
  it advances by two.

  303 + 97 + 97 = 497 = 7 * 71

  As a result, January 1 year N occurs on the same day of the week as
  January 1 year N + 400. Because the leap year pattern also recurs with
  a four hundred year cycle, a simple table of four hundred elements,
  and single modulus, suffices to determine the day of the week (in the
  Gregorian Calendar), and does it much faster than all the other
  algorithms proposed. Also, each element takes (in principle) only
  three bits; the entire table thus takes only 1200 bits; on many
  computers this will be less than the instructions to do all the
  complicated calculations proposed for the other algorithms.

  Incidental note: Because 7 does not divide 400, January 1 occurs more
  frequently on some days than others! Trick your friends! In a cycle of
  400 years, January 1 and March 1 occur on the following days with the
  following frequencies:

          Sun      Mon     Tue     Wed     Thu     Fri     Sat
   Jan 1: 58       56      58      57      57      58      56
   Mar 1: 58       56      58      56      58      57      57

  Of interest is that (contrary to most initial guesses) the occurrence
  is not maximally flat.

  In the Mathematical Gazette, vol. 53,, pp.127-129, it is shown that
  the 13th of the month is more likely to be a Friday than any other
  day.The author is a 13 year old S.R.Baxter.

  The Gregorian calendar was introduced in 1582 in parts of Europe; it
  was adopted in 1752 in Great Britain and its colonies, and on various
  dates in other countries. It replaced the Julian Calendar which has a
  four-year cycle of leap years; after four years January 1 has advanced
  by five days. Since 5 is relatively prime to 7, a table of 4 * 7 = 28
  elements is necessary for the Julian Calendar.

  There is still a 3 day over 10,000 years error which the Gregorian
  calendar does not take into account. At some time such a correction
  will have to be done but your software will probably not last that
  long!

  Here is a standard method suitable for mental computation:

   1. Take the last two digits of the year.
   2. Divide by 4, discarding any fraction.
   3. Add the day of the month.
   4. Add the month's key value: JFM AMJ JAS OND 144 025 036 146
   5. Subtract 1 for January or February of a leap year.
   6. For a Gregorian date, add 0 for 1900's, 6 for 2000's, 4 for
      1700's, 2 for 1800's; for other years, add or subtract multiples
      of 400.
   7. For a Julian date, add 1 for 1700's, and 1 for every additional
      century you go back.
   8. Add the last two digits of the year.
   9. Divide by 7 and take the remainder.

  Now 1 is Sunday, the first day of the week, 2 is Monday, and so on.

  The following formula, which is for the Gregorian calendar only, may
  be more convenient for computer programming. Note that in some
  programming languages the remainder operation can yield a negative
  result if given a negative operand, so mod 7 may not translate to a
  simple remainder.

  W = (k + floor(2.6m - 0.2) - 2C + Y + floor(Y/4) + floor(C/4)) mod 7

  where floor() denotes the integer floor function,
  k is day (1 to 31)
  m is month (1 = March, ..., 10 = December, 11 = Jan, 12 = Feb) Treat
  Jan & Feb as months of the preceding year
  C is century (1987 has C = 19)
  Y is year (1987 has Y = 87 except Y = 86 for Jan & Feb)
  W is week day (0 = Sunday, ..., 6 = Saturday)

  Here the century and 400 year corrections are built into the formula.
  The floor(2.6m - 0.2) term relates to the repetitive pattern that the
  30-day months show when March is taken as the first month.

  The following short C program works for a restricted range, it returns
  0 for Monday, 1 for Tuesday, etc.

dow(m,d,y){y-=m<3;return(y+y/4-y/100+y/400+"-bed=pen+mad."[m]+d)%7;}

  The program appeared was posted by [email protected] (Tomohiko
  Sakamoto) on comp.lang.c on March 10th, 1993.

  A good mnemonic rule to help on the computation of the day of the week
  is as follows. In any given year the following days come on the same
  day of the week:

4/4
6/6
8/8
10/10
12/12

  to remember the next four, remember that I work from 9-5 at a 7-11 so

9/5
5/9
7/11
11/7

  and the last day of Feb.

  "In 1995 they come on Tuesday. Every year this advances one other than
  leap-years which advance 2. Therefore for 1996 the day will be
  Thursday, and for 1997 it will be Friday. Therefore ordinarily every 4
  years it advances 5 days. There is a minor correction for the century
  since the century is a leap year iff the century is divisible by 4.
  Therefore 2000 is a leap year, but 1900, 1800, and 1700 were not."

  Even ignoring the pattern over for a period of years this is still
  useful since you can generally figure out what day of the week a given
  date is on faster than someone else can look it up with a calender if
  the calender is not right there. (A useful skill that.)

     References

  Winning Ways for your mathematical plays. Elwyn R. Berlekamp, John H.
  Conway, and Richard K. Guy London ; Toronto : Academic Press, 1982.

  Mathematical Carnival. Martin Gardner. New York : Knopf, c1975.

  Elementary Number Theory and its applications. Kenneth Rosen. Reading,
  Mass. ; Don Mills, Ont. : Addison-Wesley Pub. Co., c1993. p. 156.

  Michael Keith and Tom Craver. The Ultimate Perpetual Calendar? Journal
  of Recreational Mathematics, 22:4, pp. 280-282, 19
--
Alex Lopez-Ortiz                                         [email protected]
http://www.cs.unb.ca/~alopez-o                       Assistant Professor
Faculty of Computer Science                  University of New Brunswick

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