Doomsday: Odd Months

The Doomsday Algorithm for odd months 3, 5, 7, 9, and 11, i.e. March, May, July, September, and November

Added 1994-02-22, Updated 2011-02-28

Now let's do the odd months—months 1, 3, 5, 7, 9, and 11, i.e. January, March, May, July, September, and November.

Skip January and March for a moment.

Consider the following mnemonic phrase:

I work 9-5 at the 7-11

"Nine to five" is a common working day, and 7-Eleven is a chain of convenience stores. The phrase means:

This gives us Doomsday for May, July, September, and November. Now we just work our way around again within each month, using Doomsday for that month.

Example: what day is this year's July 4th?
Answer: Doomsday 2011 is Monday. The Doomsday for July (7th month) 11th is Monday. So one week earlier, July 4th is also Monday. In fact, after you do the Doomsday algorithm often enough, you just start remembering things like July 4th is always Doomsday.

Example: what is this year's Labour Day, the first Monday of September?
Answer: Doomsday 2011 is Monday. September (9th month) 5th is Monday. This is obviously the first Monday in September, since 7 days previous is still in August. SO Labour Day this year, the first Monday of September, is September 5th.

Now March.

Doomsday, the last day of February, is often also called the "0th" of March. You might have to think about that for a moment, until you realize that the next day is the 1st of March. So if the "0th" of March is Doomsday, then the 7th of March, exactly one week after the last day of February, no matter whether it's the 28th or 29th, is also Doomsday.

Example: what day is this year's St. Patrick's Day, March 17th?
Answer: Doomsday 2011 is Monday. March "0th" is Monday. March 7th is Monday. March 14th is Monday. Go three days forward, to get to Thursday, March 17th.

Finally, we have to be able to do January.

The easiest way to calculate January's Doomsday was described to me by reader Bob Goddard:

It's January 3rd three years out of four, the non-leap years. It's January 4th only in the fourth year, the years divisible by 4.

This is so much simpler than what I had before (which involved January 31st and "January 32nd"). Why couldn't I have seen the simple way?!

Example: what was this year's New Year's Day (January 1st)?
Answer: Doomsday 2011 is Monday, and since this year is not a leap year, January 3rd is Monday. Go back two days, and January 1st is Saturday. Simple, eh? Thanks, Bob.

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