What is the history behind the Friday 13th superstition?

Happy friday the 13th! I won a $100 gas card today –so far its been a GOOD day! Proves you make your own luck,but out of curiosity, some bad luck is averted, such as with throwing salt over your shoulder when you spill it, for example. Is there some way to avoid bad luck today?

heres a good link http://www.associatedcontent.com/article/200624/behind_the_superstition_friday_the.html