As you may or may not have noticed, today is Friday the 13th. But why is Friday the 13th considered unlucky? » "The modern basis for the aura that surrounds Friday the 13th stems from Friday October the 13th, 1307. On this date, the Pope of the church in Rome in Conjunction with the King of France, carried out a secret death warrant Against "the Knights Templar". The Templars were terminated as heretics, never again to hold the power that they had held for so long. There Grand Master, Jacques DeMolay, was arrested and before he was killed, was tortured and crucified.
Superstitions swirling around Friday as being lucky or unlucky have existed since ancient times, beginning with the northern nations. Ancient Romans dedicated the sixth day of the week to their beautiful, but vain, goddess Venus, so, when the Norsemen adopted the Roman method of naming days, they naturally adopted Venus as their name for the sixth day of the week. Their closest translation for Venus, Frigg, or Freya, eventually evolved into Friday, a day they considered to be the luckiest day of the week.
From a religious standpoint, Muslims tout Friday as the day Allah created Adam, legend has it that Adam and Eve ate the forbidden fruit, the apple, on a Friday, and later died on a Friday, and Christians consider Friday as the day on which Christ was crucified by the Romans.
The Scandinavian belief that the number 13 signified bad luck sprang from their mythological 12 demigods, who were joined by a 13th demigod, Loki, an evil cruel one, who brought upon humans great misfortune. The number 13, in the Christian faith, is the number of parties at the Last Supper, with the 13th guest at the table being the traitor, Judas. When Christians combine this day and number, the combination can only hold special significance."
See also Why Friday the 13th Is Unlucky at About.com.