I found another way to get the day of the week for dates…MENTALLY!
You only have to do some arithmetic in your head.
To calculate the doomsday for a given year, first find the anchor date for a century. There is a pattern that repeats every 4 centuries, here it is.
This repeats every 400 years, i.e. 1700-1799 would be Sunday, and 2200-2299 would be Friday.
After you find out the anchor date, you must find the doomsday for the specific year. An easy way to do this is the Odd+11 method.
- Let T be the last two digits of a year.
- If T is odd, add 11 to T.
- Divide by 2.
- If T is odd after division, add 11.
- Let T = 7 - (T mod 7).
- Add that number to the anchor date for the century (0 = Sunday, 1 = Monday, … 6 = Saturday).
- Convert to day of week, and you have your doomsday!
- Year: 2013
- T = 13
- T is odd, so T = 13 + 11 = 24.
- T = 24 / 2 = 12.
- T is even, so do nothing.
- 12 mod 7 = 5, so T = 7 - 5 = 2.
- The anchor date for 2000-2099 is Tuesday, so Doomsday = 2 + Tuesday (2) = 4.
- 4 mod 7 = 4, so Doomsday is Thursday in 2013.
Next, there are certain easy-to-remember dates that ALWAYS fall on the Doomsday. The full list can be found on the Wikipedia page, but some examples are the last day of February (be it the 28th or 29th), 4/4, 6/6, 7/11, Halloween, the day after Christmas, etc.
So, because Doomsday 2013 is Thursday, Halloween 2013 is Thursday. Christmas 2013 is Wednesday (Thursday - 1). Also, because April 4th is Doomsday, April 12th is Doomsday+8 (or, in terms of days of weeks, Doomsday+1), so April 12, 2013 is on Friday (Thursday + 1).
With this, you can have days of weeks all in your head, and not have to use calculators or computers. You have the power right at your fingertips. No really, there is an easy way to calculate Doomsday literally at your fingertips.
Does anybody find use in this at all?
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