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Have you ever heard of people who can calculate the day-of-week for any date in their head? How do they do it? Here we explain how anyone can be a human calendar! The basic concept is to add together 3 one-digit numbers, one for year, one for month, and one for date. Then divide the total by seven, the remainder giving a number 0 through 6 that represents Sunday through Saturday. Throw in some shortcuts, and flashcards to memorize the codes, and now anyone can look like a genius!

To start out simple, here are the directions for any date in the 1900's:

Instructions
(for 1900's)
Example:
November 12, 1970
Step 1: Determine the code for the year:

Code # 1 6 4 2 0 5 3 Year: 00 04 08 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96
Year = 70

(19)68 is 2, so 69 is 3 and 70 is 4!

Total = 4

Flashcards
Step 2: Determine month code:

 `Jan = 6` `Feb = 2` `Mar = 2` `Apr = 5` `May = 0` `Jun = 3` `Jul = 5` `Aug = 1` `Sep = 4` `Oct = 6` `Nov = 2` `Dec = 4`
Month = November

November = 2

Total = 6 (4 + 2)

Flashcards
Step 3: Determine date code:

Code # 0 1 2 3 4 5 6 Date: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Date = 12

12 = 5

Total = 11 (4 + 2 + 5)

Shortcut #1: Subtract 7, 14, 21 or 28, from the date to find code, dates less than 7 are already equal to the code.
Example:
Nov 12: 12 - 7 = 5.
Example:
Nov 26: 26 - 21 = 5

Shortcut #2: Simply add the date to the total. Example: Nov 12: Code=12, Total = 18 (4 + 2 + 12).

Flashcards
Step 4: Determine day-of-week from total

Divide the total from Step 3 by seven, the remainder giving the day-of-week:

Remainder: 0 1 2 3 4 5 6 Day-of-week: Sun Mon Tue Wed Thu Fri Sat
Total = 11 (Year=4, Month=2, Date=5)

11 divided by 7 is:
1 remainder 4

4 = Thursday

Shortcut: Instead of dividing, just subtract 7, 14, 21, 28, or 35, etc. (multiples of 7) until you have a number 0 to 6. For example, 11 - 7 = 4. If the total was 30, then 30 - 28 = 2 = Tuesday.

Flashcards
More Flashcards
Step 5: Don't forget leap-year!

Subtract 1 at any point in the process only if the year is a leap year, and the month is January or February!
1970 is not a leap year.

Shortcut #1: The year is a leap year if it is found in the table in step 1. (68 and 72 are leap years, but 70 is not)

Shortcut #2: A year is a leap year if it is evenly divisible by 4.