a year was divided into twelve mont

a year was divided into twelve months, a day. into twelve hours of daylight and twelve of night. of Twelve zodiac signs were chosen. The number thirteen destroyed this order, and was therefore considered bad luck.
Another theory goes all the way back to the Vikings who believed that it was bad luck to gather thirteen people at dinner. According to their legend, twelve gods were invited to a banquet but the god of mischief. Loki, was excluded. It made him very angry and he went to the party uninvited. He convinced tine blind kind of winter to kill their favorite god balder.