From: ts@cup.portal.com (Tim W Smith)
Newsgroups: sci.math
Subject: Re: Math Infomercial
Date: 7 Mar 91 10:53:10 GMT

Martin Gardner gave this method for calculating the day of the
week in one of his books.  It works well in your head because
there are no weird fractions involved.  It only works for
this century, though:

	1. You must memorize the following sequence:

		1 4 4 0 2 5 0 3 6 1 4 6

	(Gardner suggests thinking of this as 144, 025, 036,
	146)

	2. Take the last two digits of the year and divide
	them by 12.

	3. Add the remainder from the above division to the
	quotient.

	4. Divide the remainder from step 2 by 4 and add
	to the sum from step 3.

	5. Add the number from the sequence in step 1
	that corresponds to the current month.

	6. Add the day of the month.

	7. Reduce mod 7.  Saturday == 0, Sunday == 1, etc.

	Appendix.  For a leap year, change sequence in
	step 1 to 0 3 4 0 2 5 0 3 6 1 4 6

Example: March 7, 1991

	91/12 = 7r7.  7/4=1.  Step 4 thus gives 15.  We can reduce
	this now mod 7 to 1.

	Lookup March in the sequence from step 1.  This gives 4.
	Add the 1 from above to get 5.

	Add 7 (we want the 7th), and reduce mod 7, giving 5,
	or Thursday.

Example 2: December 7, 1941

	41/12 = 3r5.  5/4=1.  Number for 1942 is 2.
	Add 6 for December.  This gives 1.  Add 7
	because we want the 7th of the month.  This
	gives 1, or Sunday.

Adjustments for other centuries are not hard.

						Tim Smith