Mathematical poem to calculate the "day of the week" for any day of any year.

The last of Feb., or of Jan. will do

(Except that in Leap Years it's Jan. 32)

Then for even months use the month's own day,

And for odd ones add 4, or take it away*

Now to work out your doomsday the orthodox way

Three things you should add to the century day

(If you alter by sevens of course it won't matter)

In Julian times, lackaday, lackaday

Zero was Sunday, centuries fell back a day

But Gregorian 4 hundreds are always Tues.

And now centuries extra take us back twos.

*According to length or simply remember,

you only subtract for September, or November.

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Now working with Wednesday as the century day for our 1997 year, we take "Dozens, remainder and fours in the latter" from 97.

- Dozens gives us (97 DIV 12 = 8) eight twelves,
- Remainder gives us (97 MOD 12 = 1) one remainder, and
- Fours in the latter gives us (1 DIV 4 = 0) zero fours in the latter.

- Wednesday + 8 + 1 + 0 = Wednesday + 9 = Wednesday + 2 = Friday

Finally, the first part will complete the calculation in the following way:

- "The last of Feb., or of Jan. will do (Except that in Leap Years it's Jan. 32)" [Note it's not a leap year]:
- January(01) 31th is a Friday.
- February(02) 28th is a Friday.
- "Then for even months use the month's own day:
- April(04) 4th is a Friday.
- June(06) 6th is a Friday.
- August(08) 8th is a Friday.
- October(10) 10th is a Friday.
- December(12) 12th is a Friday.
- "And for odd ones add 4, or take it away* (*According to length or simply remember, you only subtract for September, or November.)":
- March(03) 7th is a Friday.
- May(05) 9th is a Friday.
- July(07) 11th is a Friday.
- September(09) 5th is a Friday.
- November(11) 7th is a Friday.

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