Furthermore, "If 13 people sit down to dinner together, one will die within the year. The Turks so disliked the number 13 that it was practically expunged from their vocabulary (Brewer, 1894). Many cities do not have a 13th Street or a 13th Avenue. Many buildings don't have a 13th floor. If you have 13 letters in your name, you will have the devil's luck (Jack the Ripper, Charles Manson, Jeffrey Dahmer, Theodore Bundy and Albert De Salvo all have 13 letters in their names). There are 13 witches in a coven." -Ask.com
It is beleived Jesus was crucified on a Friday, so many Christians beleive friday to be an unlucky day anyway
Any Friday that falls on the thirteenth day of that month is considered a day of bad luck. Additionally most tall building do not have a thirteenth floor, buildings with more than 13 floors simply skip the thirteenth (11, 12, 14, 15). This superstition dates back to the assassination of the Knights Templar ordered by the pope. Much of he Knights Templar was assassinated or arrested on Friday the thirteenth
Katharine Kurtz writes in "Tales of the Knights Templar": "On October 13, 1307, a day so infamous that Friday the 13th would become a synonym for ill fortune, 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. None of these charges was ever proven, even in France--and the Order was found innocent elsewhere--but in the seven years following the arrests, hundreds of Templars suffered excruciating tortures intended to force 'confessions,' and more than a hundred died under torture or were executed by burning at the stake."