There is a great deal of fantasy invested in the number 13 and Friday the 13th. I knew there would be information about this on the Internet and here is just one of many sources.
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. Their Grand Master, Jacques DeMolay, was arrested and before he was killed, was tortured and crucified.
I made one spelling correction in the above. Otherwise, it was taken from here: http://www.coolquiz.com/trivia/explain/ ... iday13.asp
There were 1 3/4 million other results for a search on 'basis for 13 as unlucky number' on Google.
Most of you are basing your superstitions about the number 13 upon something that you have likely never heard of such as Jacques DeMolay and the Knights Templar or of Loki as the 13th Scandanavian demi god as being evil or of the 13 'guest' at the last supper being Judas.
Well, if you like fantasy . . .