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Day For Any Date in 2009

Published on Sunday, January 18, 2009 in , , , ,

As we get 2009 going, it seems like a good time to challenge your brain with the calendar.

What better way to begin the year by memorizing the entire 2009 calendar? Don't worry, it's easier than it sounds. You can easily figure out on which day of the week any 2009 date falls by using the Doomsday method. Briefly, the last day of February, regardless of whether it falls on the 28th or 29th, is considered to be the Doomsday for that year. 2009's Doomsday is Saturday.

Using that information, it's not hard to work out any day of the week for February (February 2nd? Feb. 28th is Saturday, and so is February 7th. 5 days before February 7th is the 2nd, and 5 days before Saturday is Monday, so Feb. 2nd will be a Monday), but what about the rest of the year? The other even months are easy to remember, as 4/4 (April 4th), 6/6 (June 6th), 8/8 (August 8th), 10/10 (October 10th), and 12/12 (December 12th) will also always fall on the same day of the week as the last day of February, the Doomsday.

How about the odd-numbered months? 5/9 (May 9th) and 9/5 (September 5th) will also always fall on the Doomsday, as will 7/11 (July 11th) and 11/7 (November 7th). So, if you can remember the phrase, "I'm working 9 to 5 at the 7-11", you can remember the odd months with ease.

For the first month, January 3rd will also fall on the doomsday, unless it's a leap year, in which case you use January 4th (2009 isn't a leap year, so it's January 3rd). If you think of the last day of February as the imaginary date March 0th (Hey, it does come just before March 1st!), then you have the month of March all set, as well.

So, using the knowledge that 2009's doomsday is Saturday, how do we work out the day of the week for a given date? Let's use the date from the icon in the upper left of this post, May 1st. On what day of the week does May 1st, 2009 fall? May is the 5th month, and we know (thanks to the mnemonic "working 9 to 5 at the 7-11") that 5/9 will be a Saturday (the Doomsday for 2009). One week before that is May 2nd, which will also be a Saturday. Obviously, then, May 1st, 2009 must be a Friday!

How about Christmas 2009? 12/12 is a Saturday, and therefore so is the 26th (exactly 2 weeks later). Dec, 25, 2009, then, must fall on a Friday! July 4th is even easier - 7/11 is a Saturday, and July 4th is exactly one week before that! Can you work out on what day of the week US federal and state income taxes must be filed (April 15, 2009)? Or, if you live in the US, on what day of the week does your state's Tax Freedom Day fall?

If you want to practice this, using the Day For Any Date (Mentalist Challenge) page is a great way to do this. Just click the Get Random Date button, change the year to 2009, and the figure out that date before clicking the Show button.

Another way to check your answers for any given date would be isotropic.org's date page, although you might learn too much about any given date for your comfort (Feel free to work out the Discordian Calendar for yourself, however!).

On the go with your iPhone? You can use the iPhone Perpetual Calendar to verify dates for yourself, or use it as proof of your perfect 2009 recall when you show off your newfound ability to someone else.

Granted, it may not be as impressive as learning the full Day of the Week For Any Date Feat, but this simple method can still impress your friends, family and neighbors.

Oh, and if you already do the full version of the feat, here's a bonus tip: When you get the key number for a given year, just add 3 to it, translate that number (mod 7) into a day of the week, and you'll have the Doomsday for that year! 1978? That year has a key number of 6, to which we add 3, giving us 9. 9 mod 7 is 2, and a 2 means Tuesday, so 1978's Doomsday is Tuesday! 1936? That's 3 (the key number for 1936) plus 3, resulting in 6, and since 6 mod 7 equals 6, that 6 means Saturday is the Doomsday for 1936 (Just remember that January 4th, 1936 is a Saturday in that leap year, not January 3rd).