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Saturday, October 2, 2010

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How To Know The Day of a Particular Date

Now you can tell the day of the date(whatever it may be) that mentioned by others.

Here is a standard method suitable for mental computation:

(1) Take the last two digits of the year.
(2) Divide by 4, discarding any fraction.
(3) Add the day of the month.
(4) Add the month's key value: JFM AMJ JAS OND: 144 025 036 146
(5) Subtract 1 for January or February of a leap year.
(6) For a Gregorian date, add 0 for 1900's, 6 for 2000's,
    4 for 1700's, 2 for 1800's; for other years, add or subtract
    multiples of 400.
(7) For a Julian date, add 1 for 1700's, and 1 for every additional
    century you go back.
(8) Add the last two digits of the year.
(9) Divide by 7 and take the remainder.

Now 1 is Sunday, the first day of the week, 2 is Monday, and so on.

Comments by me:

(a) You would have to memorize the key values of Step 4.
(b) You would have to memorize the century values of Step 6 or 7.
(c) A remainder of 0 would give you Saturday at the end.
(d) You could cast out 7's as you go along if you wish, to keep the
    number small, and then step 9 is redundant.
(e) You can do steps (1 and 2), 3, 4, 5, and 6 in any order.    
    As soon as someone starts to tell you the date "July ..."
    you can do step 4, and probably reject doing step 5;
    then when he says "... 8th..." you can do step 3. When he says
    "... 19 ..." you can do step 6.  This makes it seem as though you
    have done all the calculation after hearing the date, whereas you
    have done some of it *while* hearing it.

The example of July 8, 1954, would go like this:

   (1) 54
   (2) 54/4 = 13
   (3) 13 + 8 = 21
   (4) 21 + 0 = 21
   (5) 21 - 0 = 21
   (6) 21 + 0 = 21
   (7) its for only julian date, so here it not required
   (8) 21 + 54 = 75
   (9) 75 / 7 here remainder is 5

So the day is Thursday.

Casting out 7's would give:

   (1) 54
   (2) 6
   (3) 0
   (4) 0
   (5) 0
   (6) 0
   (7) 5 <-->

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