Calculate the century number. Common century numbers are: 18XX equals five, 17XX equals three, 16XX equals one, 15XX equals zero. This pattern repeats forward, so 19XX equals zero, 20XX equals one, 21XX equals three and so forth. This pattern also repeats backward, so 14XX equals five, 13XX equals three, 12XX equals one, and 11XX equals zero and so forth. For example, when finding what day of the week May 20, 1994 was, the century number corresponding with 19XX equals zero.
Calculate the number for the year. Years range from 00 to 99. Years with zero as the year's number are 00, 04, 10, 21, 27, 32, 38, 49, 55, 60, 66, 77, 83, 88, 94. To calculate the year's number between these dates subtract the year from a year with a zero year number, then add 1 for each leap year in that span. For example for 98, is 98 minus 94 plus 1, which equals 3. 96 is a leap year, so one must be added. In the example, the number corresponding with the year for 94 equals zero.
Calculate the number for the month. The numbers for the months equal zero for a leap year February and August; one for a non-leap year February, March and November; two for June; three for September and December; four for a leap year January, April and July; five for a non-leap year January and October; and six for May. In the example, the number for the month equals six for May.
Calculate the number for the day. The number for the day is the day of the month. In the example, May 20, the number for the day is 20.
Add the number for the century, the year's number, the month's number and the day's number. In the example, zero plus zero plus six plus 20 equals 26.
Convert the number calculated in Step 5 to modulo seven. Monday equals one. Tuesday equals two. Wednesday equals three. Thursday equals four. Friday equals five. Saturday equals six. Sunday equals zero. In the example, 26 in modulo seven equals five. The fifth day of the week is Friday. May 20, 1994 occurred on a Friday.