Doomsday calendar calculation

Doomsday Algorithm was invented by Conway (1973). A year has a special day, known as a Doomsday, and dumb mnemonics tie it to a list of days within a month on that day:

> Doomsday
February 28/29 -- doomsday
> Even months
doomsday = month number
April 4 (4/4)
June 6 (6/6)
August 8 (8/8)
October 10 (10/10)
December 12 (12/12)
>Odd months
doomsday mnemonic = "I work 9-5 at the 7-11"
May 9th (5/9)
July 11th (7/11)
September 5th (9/5)
November 7th (11/7)
> January
Jan 3 in normal year, otherwise Jan 4
> March
March "0th" (aka 7, 14, 21, 28)

to recap:
1/3
2/28
3/7
4/4
5/9
6/6
7/11
8/8
9/5
10/10
11/7
12/12

For this year, the Doomsday is Sunday.

November 20, 1942

to calculate a year's doomsday:
> example: 1942
How many times does the number 12 fit as a whole into the two last digits of the year number?
> 3

What is the difference between the two last digits of the year number and the product of the multiples of 12 from calculation 1?
> 6

How many times does the number 4 fit into the result of calculation 2?
> 1

What is the century's anchor day?
> 3 (year 1900 is Wednesday)

Modulo 7 of the sum
(3+6+1+3) = 13; 13 % 7 = 6 (Saturday)

november = 7, 14, 21, 28
november 20 = november 21 (-1)
november 20 1942 = Friday

1500 1600 1700 1800
1900 2000 2100 2200
2300 2400 2500 2600
Wed. Tue. Sun. Fri