Friday, July 13, 2012

This Is The Third Friday The 13th This Year

The other two this year were in January and April.  Next year (2013) there will only be two (September and December).  Last year there was only one (May). 

So, is three a lot?  According to Paul Lutus at Arachnoid:
  • The probability that there will be a Friday the 13th during any given month is equal to the reciprocal of the number of weekdays: 1/7 or 14.1%.
  • The average number of Friday the 13ths in a year is equal to the number of months divided by the number of weekdays: 12/7 or 1.71.
So, almost 2 per year.  But according to Jim Loy, who figured the 1/7 figure as well,
It turns out that this is not quite true. It was shown by Brown (I don't know his first name) in 1933 that the Gregorian Calendar (which we use) repeats itself exactly, every 400 years. In that time, there are 4800 months and 4800 13ths. Of those 4800 13ths, 688 occur on Friday. So the probability of a Friday the 13th is 688/4800 which is .143333..., which is slightly greater than 1/7. In fact, Friday is the most likely 13th, slightly. Of the 4800 13ths, Sunday is the 13th 687 times, Monday 685, Tuesday 685, Wednesday 687, Thursday 684, Friday 688, and Saturday 684.


  1. I'm working on a daily art project in which I'm arranging my pieces like a calendar, seven across. I'm making four panels, one for each quarter. This year three of the four quarters start on Sunday, which means I have nice square corners at the top of three of the four panels. I think that's a nice anomaly too. Haven't figured out the odds, but it needs to be a leap year.

  2. Kathy, Check out which months those are. For Friday to be on the 13th, the month has to start on a Sunday. So, I guess the Friday the 13ths are all in the first month of the quarter.

    Do we get to see a picture of the panels?


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