Doomsday: Even Months

The Doomsday Algorithm for even months 4, 6, 8, 10, and 12, i.e. April, June, August, October, and December

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

Okay, the last day of February is Doomsday. Once we know what day of the week Doomsday is, we immediately know the day of the week of certain other days in the year. There are actually 52 (or 53) other Doomsdays each year, all on the same day of the week as "the" Doomsday at the end of February.

So each month has a special day which we remember, because it is on the same day of the week as the Doomsday at the end of February. Just keep in mind that the entire year is determined by the Doomsday at the end of February, and that all the other Doomsdays within the year are on the same day of the week.

Let's begin with the even months. These are months 2, 4, 6, 8, 10, and 12, i.e. February, April, June, August, October, and December. Actually, we never do February this way, because it's special, and we've already covered it.

For even months (not including February), the Nth of that month is a Doomsday (i.e. the same day of the week as the last day in February). This is a delightful coincidence, and it's so easy to remember:

Neat, eh? Now we can simply work our way around any even month based on its Doomsday.

Example: what is this year's Christmas Day, December 25th?
Answer: Doomsday 2011 is Monday. So December (even month) 12th is the Doomsday for December, so it's also Monday. Two weeks later, December 26th is also Monday, so Christmas this year is Sunday December 25th. Easy! In fact, after you do the Doomsday algorithm often enough, you just start remembering things like Christmas is always the day before Doomsday.

Example: what is this year's Canadian Thanksgiving Day, the second Monday in October?
Answer: Doomsday 2011 is Monday. So October (even) 10th is Monday. This has to be the second Monday of October, since a week earlier is October 3rd, but a week earlier than that is in September. So the Canadian Thanksgiving, the second Monday in October, is Monday, October 10th.

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