Doomsday: Odd Months

The Doomsday Algorithm for odd months 1, 3, 5, 7, 9, and 11, i.e. January, March, May, July, September, and November

Added 1994-02-22, Updated 2016-01-04

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 (9 a.m. to 5 p.m.) while 7-Eleven is a chain of convenience stores. This mnemonic phrase means:

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

Example: what day is this year's July 4th?
Answer: Doomsday 2016 is Monday, so the Doomsday for July (7th month) is the 11th, also 6a 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 2016 is Monday. September (9th month) 5th is Monday. In other years, we would then need to do some more to get from Doomsday to Monday, but in 2016 we're done, since Doomsday is Monday, and it's obviously the first Monday in September, since 7 days previous puts us 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 2016 is Monday, which we know is the "0th" of March. So a week later, March 7th is Monday. March 14th is Monday. Now we go three days forward, to get to Thursday, March 17th.

An alternate method for March is to use Pi Day, which is March 14th, i.e. 3/14 using month/day numbers. Pi Day is always Doomsday.

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 is this year's New Year's Day (January 1st)?
Answer: Doomsday 2016 is Monday, and since 2016 is a leap year, January 4th is Monday. Go back 3 days, and January 1st is Friday. Simple, eh? Thanks, Bob.

Another way to calculate January's Doomsday was sent to me by reader Roman Weil. It's actually due to his son Sandy Weil, who is the Director of Football Analytics for the Baltimore Ravens:

A January trick: Instead of associating January with the new year, associate it with the old. That is, think of Jan 2016 as being part of 2015. In that case, Pi Days in January are 1/2 and 1/23. So in 2016, 1/2 and 1/23 are Saturdays, the Pi Day of 2015. You will find, if you are like me, that when you think about January, it's more often about 'next January' than about 'last January,' so putting January at the end of the current year will solve most of your January issues.

Sandy here refers to "Pi Days" which is another name for Doomsdays—this is further discussed in Origins. The mnemonic part of Sandy's trick is that 1/2 and 1/23 sound like "one two" and "one two three."

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