Friday the 13th

The fear of Friday the 13th is called paraskavedekatriaphobia or paraskevidekatriaphobia, a specialized form of triskaidekaphobia, a phobia (fear) of the number thirteen.
Thirteen may be considered a "bad" or "unlucky" number simply because when a group of 13 objects or people is divided into two, three, four or six equal groups, there is always one leftover, or "unlucky", object or person.

The number 13 is probably the most common of all superstitions, considered a bad luck day unless you were born on Friday the thirteenth in which case it is allegedly your lucky day.

It was suggested by Charles A. Platt writing in 1925 that the reason 13 is considered unlucky is that a person can count from 1-12 with their 8 fingers, two thumbs and 2 feet, but not beyond that, so the number 13 is unknown, hence frightening, hence unlucky. This idea discounts the use of toes or other body parts in counting.

In Scotland, there is no gate 13 in any airport, instead there is a gate 12B.

Inside of a Lufthansa plane with row numbers going straight from 12 to 14.

Some airplanes skip a row 13, going straight from 12 to 14.

Some tall buildings have resorted to skipping the "thirteenth floor", either by numbering it "14" or as "12a".

Some streets do not contain a house number 13.

In some forms of motor sport, for example Formula One, there is no number 13 car.

In many cultures, getting married on any day of the week that falls under number 13 is highly discouraged.

There is a superstition that should thirteen sit at a table to dine, one will die in the next year.