0

## Friday the 13th

Published on Sunday, November 27, 2005 in , , ,

Thanks to a recent e-mail discussion with some friends who also share an interest in memory work, we've managed to devise a simple way to find out when and how frequently "Friday the 13th" will occur in a given year.

First, you'll need to be able to perform the standard calendar feat. There are several variations in the formula, so I'll assume that you're using the version taught here. Once you've become comfortable practicing this version, you'll be ready to move on to the Friday the 13th version of the feat.

Start with the number 6. From this, subtract the code number for the year. You're done.

Yes, that's it. The resulting number will give you the month code for all months in that same year that contain a Friday the 13th.

As an example, let's figure out in which months a Friday the 13th will occur in 2006. The key number for 2006 is, interestingly, 6 (See here and here for details on how this was determined). Subtracting 6-6, we get 0.

Which months have a code number of 0? Only January and October do. Therefore, in 2006, only January and October will have a Friday the 13th. Checking the 2006 Calendar, we can verify that this correct.

Don't forget to adjust for leap years when necessary. In leap years, January is 6 and February is 2.

Why does this work? The original calendar formula has you add up numbers representing the date, month and year to get the day of the week, so it stands to reason that we can start with the day of the week, and subtract the date and year to obtain a month number.

Friday is equal to 5. Keeping in mind that we can add multiples of 7 without affecting the outcome, we'll add 14 to 5, getting 19. From this, we can subtract the date in which we're interested, the 13th, without working with negative numbers. 19 minus 13 equals 6. From 6, all we need is the key number for the year, which runs from 0 to 6. The result can only be the key number for the months containing a Friday the 13th!