Friday the 13th is most common 13th

January 13, 2012 By: erik Category: Fighting Stupidity, Geeky, Math, Musings 732 views

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Friday the 13th Facts and TheoriesI’ve always been fascinated by superstition, and friggatriskaidekaphobia – or, to be more clear, paraskevidekatriaphobia – strikes me as a particularly interesting one. The origin can only be traced back into the 19th century. I am disappointed to discover that experts find little reason to associate it with the slaughter of the Knights Templar on October 13, 1307, exactly seven hundred years before my wedding day. Oh well, something else Dan Brown got wrong. As if to show just how arbitrary the choice of Friday is, the Spanish speaking world fears Tuesday the 13th, and they even have their own tongue-twisting phobia word: trezidavomartiofobia.

Several months ago, one of my journeys down the rabbit hole of Wikipedia found me reading more about Friday the 13th, and I was fascinated to discover that, of all the days of the week, the thirteenth day of any given month is statistically more likely to be a Friday.

How was this discovered? The Gregorian calendar – the one we use in modern 21st century western society – repeats itself every 400 years. Therefore, to calculate how many times the thirteenth falls on each day of the week, we only have to examine 400 years, or 4800 months. A bloke by the name of B.H. Brown did this by hand in 1933, but now we have computers. The program to do this is completely trivial once you have a calendar algorithm to follow the Gregorian calendar like all modern computer languages have built-in. Trivial or not, I couldn’t resist…let’s calculate how many times the 13th falls on each day of the week, shall we?

13th Histogram Calculator

When you click “GO!”, the calculator will start from today, January 13, 2012, and count which weekdays fall on the thirteenth of the month over the next 400 years until January 13, 2412. Here’s a hint: it goes up to 688.

Sunday 0
Monday 0
Tuesday 0
Wednesday 0
Thursday 0
Friday 0
Saturday 0

Exciting, huh? There was only one Friday the 13th in 2011, in May, so I've been waiting months to post this.

  • Anonymous

    I’m glad you didn’t have to do that by hand. 😉