- Pick a base year and find out (with a perpetual calendar, probably) what day of the week January first was in that year. I use 1900, but you can pick pretty much any year you want.
- Count the number of years since that year. Since there are 365 days in a year, and 365 = 52 * 7 + 1, a month/day combination shifts forward one day per year.
- Count the number of leap years (including the current year) since the base year (include the base year, if applicable). Every leap year of course adds one extra day. Note the rule for determining whether a year is a leap year, below.
- Add in a "magic" number that offsets for the month within the year. It's not really magic, but it can seem that way. What you do is assign a number to January that corresponds to the day of the week it started on in your base year. I want my answer to come out as a number between 0 and 6 (inclusive), with 0 indicating Sunday. Since 1900 began on a Monday, I give a 1 to January. Then you can compute the "magic" values for the remaining months by simply adding the number of days in the current month (28 for February; we already took care of leap years in step 3) to the number for the current month and computing the result modulo 7. (See below if you don't know modular arithmetic.) So my numbers are 1, 4, 4, 0, 2, 5, 0, 3, 6, 1, 4, 6.
- Add in the day of the month.
- If the date in question is in a leap year, but before February 29th, we added in an extra day in step 3, so subtract one.
- Compute the modulus-7 value of the sum of the numbers in steps 2-6.
- You now have a value between 0 and 6, inclusive. 0 indicates Sunday, 1 indicates Monday, etc.

- Divide the last two digits of the year by 12 with integer division, getting a quotient and a remainder. Come on now, you did this in grade school - it's not hard. Add those two numbers, modulo 7.
- Divide the remainder found in step 1 (not the sum, the remainder from the division) by 4 and get the quotient. (We don't need this remainder.) Add it to the sum found in step 1 and again take the modulus-7 result.
- Add in the value assigned to the desired month:
January 1 February 4 March 4 April 0 May 2 June 5 July 0 August 3 September 6 October 1 November 4 December 6 If you are in to math and recognize the three-digit patterns of perfect squares and that helps you remember this step, use that. Otherwise, just memorize those numbers or memorize how they were created (see step 4 in the general algorithm).

Compute the modulus-7 result.

- Add in the day of the month and compute the modulus-7 result.
- If you are in a leap year but before February 29th, subtract one, modulus 7.

You can shift it to 1800 by changing the magic numbers for the months. That year, 1 January was a Wednesday, so add 2 (modulo 7) to the numbers. 1 Jan 1700 was a Friday, but for dates prior to 14 Sep 1752, things get messy anyway, since that was when the British government added days to the calendar to account for the new information about the length of the year. (14 Sep 1752 immediately followed 2 Sep 1752.)

If you aren't sure, keep track of where you are at every step of the way. It will be printed out on the answer page.