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:
November 12, 1970
|Step 1: Determine the code for the year:
|Year = 70
(19)68 is 2, so 69 is 3 and 70 is 4!
Total = 4
|Step 2: Determine month code:
|Month = November
November = 2
Total = 6 (4 + 2)
|Step 3: Determine date code:
|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.
Nov 12: 12 - 7 = 5.
Nov 26: 26 - 21 = 5
Shortcut #2: Simply add the date to the total. Example: Nov 12: Code=12, Total = 18 (4 + 2 + 12).
|Step 4: Determine day-of-week from total
Divide the total from Step 3 by seven, the remainder giving the day-of-week:
|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.
|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.
Click here for the rare exceptions to the above shortcuts.
There you have it! Actually it may take alot of practice before you become proficient, but don't give up! Almost anyone can be a human calendar! Click here for the flashcards to practice the whole process for the 1900's.
Okay, so you have mastered calculating the day-of-week for any date in your head, but only for the 1900's? Click here to show the process when the date is anywhere between 1800 and 2199.