The thirteenth day of the month is considered "unlucky" if it falls on a Friday, why? That's because 13 was considered a very unlucky number. In Scotland, Thirteen is known as the "Devil's Dozen"--a title characteristic of the worst associations of this much abused number. The Romans considered that the fatality followed the number whenever and for whatever purpose thirteen people gathered together. The superstition surrounding the number 13 could also be linked to Norse mythology. According to legend, 12 gods were at a banquet at Valhalla when Loki, the god of mischief who was not invited, turned up, bringing the total number of guests to 13. He was responsible for the chaos that led to the death of one of the gods so all the gods grieved.
And what about Friday? One reason for this has something to do with the the alleged date of Jesus' crucifixion which was a Friday, but the belief in its ill luck probably goes much farther back in history and may have something to do with the sacrifices offered to the goddess Friga (or Frigg, Frigga)in Norse mythology. She is said to be the wife of Odin, and is the "foremost among the goddesses".
Other thinks that the unlucky day was connected on October 13, 1307. It was the day when officers of King Philip IV of France carried out mass arrests in a well-coordinated dawn raid that left several thousand Templars — knights, sergeants, priests, and serving brethren — in chains, charged with heresy, blasphemy, various obscenities, and homosexual practices.
It is a tradition in Britain that Friday was the conventional day for public hangings, and there were supposedly 13 steps leading up to the noose.