"The belief that Friday 13th is an unlucky day is one of the most widely know superstitions in Britain today.

The superstition surrounding Friday the 13th is actually a combination of two separate fears — the fear of the number 13, called triskaidekaphobia, and the fear of Fridays.

Unlucky 13

It is unlucky to have thirteen at a table or in a company. The number 13, in the Christian faith, is the number of people at the Last Supper, with the 13th guest at the table being the traitor, Judas.

(The unlucky number 13 is often left out when numbering houses, flats and hotel rooms. )

Fridays

Fridays have been considered an unlucky day since the medieval times.

In Britain, Friday was the conventional day for public hangings, and there were supposedly 13 steps leading up to the noose!

Adam and Eve ate the forbidden fruit on a Friday and later died on a Friday.

Christians consider Friday as the day on which Christ was crucified by the Romans."

http://www.woodlands-junior.kent.sch.uk/customs/questions/friday13.htm

So I guess, in summation - 13 at the Last Supper and Friday was the Crucifixion.....