Its trivial to determine the day of the week from the date if you just remember this number:
0336 1462 5035. (Month Code).

Take any date

month day
+ month code
+ last two digits year
+ integerpart(last two digits year/4)
-1 (century code).

Then divide by 7 and find remainder. 0=Sunday, 1=Monday, 2=Tuesday etc etc.

e.g. 22nd May 2010.

22+1+10+2-1=34
remainder 6 after dividing by 7 so this was a Saturday.

e.g. 14th March 2007.

14+3+7+1-1=24
remainder is 3 after dividing by 7 so this was a Wednesday.

For dates in the 2000's we subtract one from the end. But its even easier for the 1900's as we don't need to subtract anything. eg.

20th July 1987

20+6+87+21 = 134
remainder is 1 when dividing by 7 so the day was a Monday.