Happy Doomsday, everyone! Did you know that the last day in February is called “Doomsday”? not in a “the world is ending” way, but it’s a mathematical equation called the “doomsday rule”, figured out by mathematician John Conway. You can read all about it on places like Wikipedia, but the jist is this: Doomsday is just a name, he could have called it “Conway Day”, but Doomsday has more punch! Doomsday is the last day of February each year. (a.k.a. March 0, or ZEROth of March).

What day does Christmas fall on in 3 years?

Where this rule is helpful is finding out what day of the week a certain date, holiday, birthday, etc. falls on. You might be thinking: I can always just Google it, or search for it on my calendar. Yes…IF you are at a computer. But what if you are not at a computer? What day is Christmas in 2015? Well, its on a Friday, of course! Isn’t that a great day for Christmas, because you probably get the Eve off, and then Boxing day is a Saturday – Sunday is an extra day. Boxing day always falls on Doomsday. So does Halloween, and the 4th of July, and that’s just for starters. Read on!

How to remember the rule { impress your friends }

These dates also fall on Doomsday (remember even numbers after 2): 4/4, 6/6, 8/8, 10/10, and 12/12. For the odd months these dates are also on Doomsday: 5/9, 9/5, 7/11 and 11/7 (remember “Seven Eleven is open from Nine to Five”). There you have all the months except January. For January, in regular years it is 1/10. Think of the tenth as 1 year, 0 extra days. On leap year it is 1/11 – 1 year, 1 extra day.

In normal years (the next three) Doomsday advances by one day (365 is 52 weeks + 1 day), and leap years it advances by two. This may seem like a lot to remember, but once you know the Doomsday for one year, you get used to figuring ahead. This year, 2012, doomsday is on Wednesday, so the next three years are Thursday, Friday, Saturday. So, in 2014, July 4th is on a Friday – a good day for that holiday.

But wait, there’s more.

*Warning – for the extra-math-geeky only! Yes, there is a whole algorithm for figuring out any year in any century, but for most people, knowing a few years ahead (and the current year, of course) can be helpful. If you really want to know what day will be New Year’s in 2099, you can do the math.