Friday, 25 July 2014

It’s easier than you might think to calculate the day of the week for a date from September 14th, 1752 (the day the Gregorian calendar was made official in America and England) to the indefinite future.
The days of the week are numbered as follows:
Sunday    1
Monday    2
Tuesday   3
Wednesday 4
Thursday  5
Friday    6
Saturday  0
For each month there is a key value:
January   3
February  6
March     6
April     2
May       4
June      0
July      2
August    5
September 1
October   3
November  6
December  1
And finally, the century values:
1700's 2
1800's 0
1900's 5
2000's 4
2100's 2
Now you will be keeping a running total of a series of numbers. First, take the last two digits of the year as your initial number. Divide that number by 4 and discard the remainder. Add the result to your initial number. Then add the century value. Next add the key value for the month in question. Then add the day of the month. Now divide the grand total by 7. The remainder is all that counts.
Example #1:
Suppose we want to know what day of the week December 25th, 2006 is.
Last two digits of year: 6
Divided by 4 (discarding remainder): 1
Century Value: 4
Key Value for December: 1
Day of Month: 25
—-
Total: 37
37 / 7 = 5 with a remainder of 2 so our answer is Monday.