Christmas Day

So, what day of the week does Christmas day fall on?

1. Select the year you are interested in. Break this into two parts: Century and Year.
   example: 2001 becomes C = 20, and Y becomes 01.
2. Divide C by 4 and truncate. Store the result into K.
   example: K = C/4 = 20/4 = 5
3. Divide Y by 4 and truncate. Store the result into G.
   example: G = Y/4 = 1/4 = 0
4. D = 50 + Y + K + G - (2 * C)
   example: 50 + 1 + 5 + 0 – 40 = 16
5. Take the remainder of D/7 (or take the modulo: D % 7) example: 16 % 7 = 2
6. Lookup:
   • 0 Sunday
   • 1 Monday
   • 2 Tuesday
   • 3 Wednesday
   • 4 Thursday
   • 5 Friday
   • 6 Saturday

Please note that this applies only to the Gregorian calendar that was first introduced in 1582.