Doomsday: February 28 or 29

The last day of the February, either the 28th or the 29th, is known as "Doomsday"

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

To use the Doomsday Algorithm in any year, we first need to know the Doomsday for that year.

Doomsday is February 28 or 29. In other words, Doomsday is always the last day of February. In normal years, Doomsday is February 28, and in leap years, Doomsday is February 29.

In 2011, which is not a leap year, the last day of February is Monday the 28th.

Once we know Doomsday, it's pretty easy to get the day of the week for any day in February. This is done by adding and subtracting, using multiples of 7, and you should be comfortable doing this in your head, otherwise the rest of the algorithm will give you trouble! Luckily, most people, through practice or whatever, are good at mentally picturing a month if they have something to anchor it on, and Doomsday is this anchor. For February, it's always the 28th in normal years, and the 29th in leap years.

Example: what was this year's Valentine's Day, February 14th?
Answer: Doomsday 2011 is Monday the 28th of February. So one week earlier, the 21st is also Monday. Another week earlier is Monday the 14th. So Valentine's Day 2011 is a Monday.

Example: what was this year's Groundhog day, February 2nd?
Answer: Doomsday 2011 is Monday the 28th... Monday the 21st... Monday the 14th... Monday the 7th... and then we have to go five days back, to get from the 7th to the 2nd. So Groundhog Day, February 2nd, is Wednesday this year.

If going back five days is hard (and it often is, especially looking back over a weekend), there's a little trick we can use here. Going two days forward gives the same day of the week as five days back. So if February 7th is Monday, then two days forward is Wednesday the 9th, which is the same day of the week as seven days earlier, Wednesday the 2nd. Remember, all we're after is the day of the week, so "-5" is the same as "+2" but "+2" is usually easier to do.