MenuHave 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 |
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Step 1: Determine the code for the year:
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Year = 70 (19)68 is 2, so 69 is 3 and 70 is 4! Total = 4 |
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Step 2: Determine month code:
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Month = November November = 2 Total = 6 (4 + 2) |
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Step 3: Determine date code:
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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). |
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| Step 4: Determine day-of-week from total Divide the total from Step 3 by seven, the remainder giving the day-of-week:
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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. More Flashcards |
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| 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. |
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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.