Several different reasons exist for the superstition that the number 13 brings bad luck, such as the fact that 13 people sat down at the Last Supper, shortly before the crucifixion of Jesus. The Norse legends included a story about 12 gods sitting at a banquet when Loki, the uninvited god, showed up to make it 13, a debacle that led to a huge battle. Numerologists view the number 12 as the sign of completion and perfection, so adding to it only causes trouble.
Other superstitions have grown up around the number 13, of course. One belief was that a coven of witches had to contain exactly 13 members, and another suggested that if your name has 13 letters, you are fated to have the luck of the Devil. Charles Manson, Jeffrey Dahmer and Jack the Ripper might agree, of course.
Friday and the number 13 have developed a special relationship when it comes to bad luck. Part of this dates back to the mass roundup and execution of members of the Knights Templar, which took place on Friday, October 13 in the year 1307. Friday, October 13, 1972, was the day when a Uruguayan plane crashed along the Andes Mountains, killing 29, and a Russian plane crashed about half a mile away from the runway, killing 174. So the superstitious out there have plenty of ammunition.