Calculate the Day of the Week

A date between January 1st 1900 and December 31st 2099 is displayed. Guess the day of the week.

How to make the calculation ?

Example: 17 / 12 / 1997
1) remember the day of the month (17)
2) remember the "code" for the month (Jan=6;Feb=2;Mar=2;Apr=5;May=0;Jun=3;Jul=5;Aug=1;Sep=4;Oct=6;Nov=2;Dec=4) In this example we remember (4)
3) Remember the year (97 - without the century 19)
4) divide the year 97 by 4 and forget the number after the comma: 97:4=24,25 ==> we remember (24)
5) add 1 for all dates between 1/1/1900 and 31/12/1999
6) check if it was a leap year - if it was a leap year : the code for Jan becomes 5 and for Feb it becomes 1
7) add all remembered number together: 17+4+97+24+1 = 143
8) distract the largest possible multiple of 7 => 143-7*20 = 143-140 = 3
9) 3 stands for wednesday (1 for monday; 2 for tuesday, ...)

Don't give up until you reach an average of 10 seconds - after a while it becomes easy, especially when you learn how to think in a seven numerical system.