Next Friday the 13th

This countdown targets the next Friday the 13th. Astronomically the date is unremarkable: it is a calendar coincidence, occurring one to three times a year. No year is without one, and none has more than three.

Why never zero? Because the Gregorian calendar repeats on a 400-year cycle, and within that cycle each month begins on a given weekday a known number of times. The arithmetic yields a counter-intuitive result: across the 4,800 months of a cycle, the 13th falls on a Friday more often than on any other day of the week.

Where the superstition comes from. Two older fears stacked together. Thirteen has a poor reputation in the Christian world — thirteen at the table at the Last Supper — and Friday is the day of the Crucifixion. Their fusion into a single unlucky date is recent, though: there is little trace of it before the nineteenth century. The arrest of the Knights Templar on Friday 13 October 1307 is often cited, but the connection to the superstition was made long afterwards, popularised largely by a twentieth-century novel.

The fear has a name: paraskevidekatriaphobia. Some buildings still skip the thirteenth floor, and some airlines the thirteenth row.

Elsewhere the unlucky day differs. In Spain and Greece it is Tuesday the 13th; in Italy, Friday the 17th.