Why is the number 13 believed to be unlucky?

 It is just another superstition but some people take it seriously. Many hotels don’t have a 13th floor and most people take care not to have 13 persons at a dinner table. Perhaps it has something to do with the Last Supper in the Christian religion or the story of the Valhalla banquet in Scandinavian mythology, at which the 13th visitor, Loki, the spirit of strife, caused mayhem.