It’s Friday the 13th and we have a special Second Fridays event (Loving Lititz Every Second) in our little town.  I wondered about the probability of a second Friday falling on Friday the 13th. Thought I might have a contest and offer a cash prize to the first person who could figure those odds.

Turns out I couldn’t figure those odds. I started by thinking that any randomly selected month would have the probability of a Friday the 13th as  1/7 odds from the seven days of the week). But leap year complicates that, making a 28 year cycle until the calendar repeats. Then I learned that the Georgian calendar, which we use, only repeats every 400 years. What’s up with that?

Turns out that 365.25 days per year doesn’t quite work. It’s actually a 400 year cycle with year 100, 200 and 300 not being leap years, unlike year 400. So year 2000 had a leap year, but 1900 and 2100 don’t.

So figuring the odds goes from a simple probability problem that I was anticipating to creating a table and inputting dates. Then the odds get determined by counting the numbers.  I think folks in construction should have a basic knowledge of simple probability and odds, it just helps you make better decisions.

Thinking of rational decisions, what’s your opinion of Friday the 13th? Wikipedia had some interesting info:

The fear of Friday the 13th is called friggatriskaidekaphobia (Frigga being the name of the Norse goddess for whom “Friday” is named and triskaidekaphobia meaning fear of the number thirteen), or paraskevidekatriaphobia^[1][2] a concatenation of the Greek words Paraskeví (Παρασκευή, meaning “Friday”), and dekatreís (δεκατρείς, meaning “thirteen”) attached tophobía (φοβία, from phóbos, φόβος, meaning “fear”).

As in, “Take it easy on me today, baby, I’ve got Friggatriskaidekaphobia.”