How to Calculate Days of the Week Like Rainman

Step-by-Step Guide

1. 1

   Step 1: First we need a number for every day of the week: Monday is

   There are two ways to do it so I’ll show the first and then I’ll show the one that I think is easier.
   
   Once you deal with these you will be able to easily take a date and mentally crunch the day of the week rapidly in your head just like Rainman. , For example: 2000 = 0 2001 = 1 2002 = 2 2003 = 3 2004 = 4 2005 = 5 2006 = 0 Etc.
   
   Of course this is problematic because a “Leap Year” occurs every four years so that has to be factored in or not.
   
   Leap Years occur every four years, 2008 was a leap year so: 2007 = 1 2008 = 3 2009 = 4 2010 = 5 2011 = 6 2012 = 1 2013 = 2 Notice the zero got moved around.
   
   The good news is there’s a shortcut, we’ll take the shortcut.
   
   Just take the last two digits of any year and divide it by four and ignore the remainder. (Anytime you don't have a remainder it's a Leap Year and then January Code is 5 and February Code is 1 as shown above) For Example:
   The year 2061 is handled as, (61 / 4) = 15 with a remainder of one (Discard the one), Then add that answer to the last two digits of the year (61 + 15) = 76 ,, Divide 10 by 4 and we get 2 with a discarded remainder of
   2.
   
   So we add 10 + 2 for an answer of 12 and see that a multiple of 7 will give us a 5 left over so the year code for 2010 is “5”. ,, That’s; (5 + 25 + 4) =
   34.
   
   Then multiples of seven will get us to (7 * 4) =
   28.
   
   The (34 – 28) =
   6.
   
   The number six is the code for Saturday.
   
   So Christmas day 2010 will fall on a Saturday. , Month Code; Divide the last two digits of a year by four, discard the remainder and add the whole number to the last two digits of the year.
   
   Then use the multiples of 7 to get as close as you can to that answer and subtract the two for the year code. ,
2. 2

   Step 2: 1 Tuesday is

3. 3

   Step 3: 2 Wednesday is

4. 4

   Step 4: 3 Thursday is

5. 5

   Step 5: 4 Friday is

6. 6

   Step 6: 5 Saturday is

7. 7

   Step 7: 6 Sunday is

8. 8

   Step 8: 0 or 7;

9. 9

   Step 9: Have a number for every month: January is

10. 10

    Step 10: 6* February is

11. 11

    Step 11: 2* March is

12. 12

    Step 12: 2 April is

13. 13

    Step 13: 5 May is

14. 14

    Step 14: 0 June is

15. 15

    Step 15: 3 July is

16. 16

    Step 16: 5 August is

17. 17

    Step 17: 1 September is

18. 18

    Step 18: 4 October is

19. 19

    Step 19: 6 November is

20. 20

    Step 20: 2 December is

21. 21

    Step 21: 4 For a Leap Year * January is 5 and February is 1 but that will be explained shortly.

22. 22

    Step 22: Have a code for every year

23. 23

    Step 23: which is simple as well but for now let’s put it together and see the formula at work: Let’s use Christmas 2009; December Month Code (4) + Day (25) + Year Code (4)

24. 24

    Step 24: which is: (4 + 25 + 4) = 33.

25. 25

    Step 25: Now get as close to the 33 as you can with multiples of seven

26. 26

    Step 26: subtract the difference and then use the remainder for the week code

27. 27

    Step 27: (7 * 4) = 28

28. 28

    Step 28: then (33 – 28) = 5

29. 29

    Step 29: and that’s it

30. 30

    Step 30: five is the day code for Friday

31. 31

    Step 31: so Christmas day this year will occur on a Friday.

32. 32

    Step 32: Before we go any further let’s look at how the Year Code is calculated.

33. 33

    Step 33: Starting with the year 2000 we assign the code 0 and then add one to each successive year.

34. 34

    Step 34: Now we need to use multiples of seven to get as close to that answer as we can so we’ll use (7 * 10) = 70

35. 35

    Step 35: now subtract the two numbers: (76 – 70) = 6 That’s it; the year code for 2061 is 6.

36. 36

    Step 36: Again take 2010.

37. 37

    Step 37: Let’s see what day of the week Christmas will fall on next year?

38. 38

    Step 38: Year Code (5) + Day (25) + Month Code (4).

39. 39

    Step 39: Recap The formula is Month Code + Day + Year Code and then use multiples of seven to get as close as you can to the answer and then the remainder is the Day Code that gives the Day.

40. 40

    Step 40: We could add one more step to that and calculate backwards to see what day the Declaration of Independence was signed but we need to leave Rain man something to do.

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