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 2017-01-03

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 days which are all on the same day of the week as "the" Doomsday at the end of February, but there's a special one each month which we will now learn.

Each month has a special day which we remember, because it is on the same day of the week as the Doomsday which is at the end of February. We call these the Doomsdays for their months. 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. In other words, it's 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 2017 is Tuesday. Then December (even month) 12th is the Doomsday for December, so it's also Tuesday. Two weeks later, December 26th is also Tuesday, so Christmas, the day before, is Monday 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 Halloween, October 31st?
Answer: Doomsday 2017 is Tuesday. So October (even) 10th is Tuesday. Then three weeks (21 days) later is Tuesday, October 31st. Easy! In fact, after you do the Doomsday algorithm often enough, you just start remembering things like Halloween is always Doomsday.

Example: what is this year's Canadian Thanksgiving Day, the second Monday in October?
Answer: Doomsday 2017 is Tuesday. So October (even) 10th is Tuesday. A week earlier is Tuesday October 3rd, and so the day before, Monday the 2nd, is the first Monday of October this year. So the Canadian Thanksgiving, the second Monday in October (also Columbus Day in the US), is Monday, October 9th in 2017.

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