Why 13 is famous as Unlucky?
* Alexander wanted to be a God and accordingly made a 13th statue on the place of his capital. He died and people considered that his will to be a 13th God (12 gods, one for each month were known) was the reason of his death. After that this number was considered as unlucky.
* According to historians, there were 13 people at Christ's Last Supper and Christ was crucified on Friday 13th. So, Friday 13th is considered as Unlucky.
http://www.shyamsundergupta.com/unlucky13.htm