Durwood says you can mentally calculate the day of the week for any day in the year 2000 very easily. Who cares if the computers can't tell what day it is? You can figure it in two steps, with the answer being the day of the week (Sun = 1, Mon = 2, etc).

Step one: add the day of the month to the month number and subtract one. (You'll have to memorize the month numbers below.) Step two: Subtract the biggest multiple of 7 that you can. (Or divide by 7 and use the remainder.)

Here are the month and weekday numbers.
 

Month      Number     Day         Number
January      0        Sunday        1
February     3        Monday        2
March        4        Tuesday       3
April        0        Wednesday     4
May          2        Thursday      5
June         5        Friday        6
July         0        Saturday      7 or 0
August       3
September    6
October      1
November     4
December     6

Day of month + Month No. - 1 = Day of Week (correct for multiples of 7)
So for Jan. 1 you have (1 + 0 - 1 = 0 = Sat)
For April 10 use (10 + 0 - 1 = 9 , then 9 - 7 = 2 = Mon)
For your birthday _____________________________

Is Thanksgiving the 25th? (25 + 4 - 1 = 28 7 = 0 remainder = Sat). No! Thursday would be the 23rd (counting backwards) = 4th Thursday since 21 days = 3 weeks. The 30th would be the 5th Thursday, but not Thanksgiving.

When is Christmas ? (25 + 6 - 1 = 30 - 28 = 2 = Mon)

After you get this down, you can do slightly different things for other years. 2000 has a year number of 0 and a century number of -1 (or +6). (That's the -1 we used.) Years in the 1900's have century numbers of 0. 1800's are +2, and 1700's are +4.

Year numbers within a century go up by 1 each year and each leap year. 2000-to-2003 all have a leap year number of 0, and have year numbers of 0, 1, 2, and 3 because 365 days has one day over 52 weeks . You can divide the last 2 year digits by 4, discard the remainder, and add the leap year number to the last two digits. 1999 had a leap year number of 24 to add to the 99 days (over 52 weeks) that accumulated all century. (=123 7 =remainder 4 = year number).

Month numbers change on leap years, but we only change January and February. They are 1 and 4 in normal years and 0 and 3 in leap years. Month numbers represent the cumulative days over 4 weeks in the previous month .