Superior Squid
It has to do with myths and math. See, the number 12 is supossedly perfect. 12 Greek gods, 12 months in a year, 12 hours in a day, 12 hours in a night, etc. The number 13 is believed to be imperfect. And many people are afraid of imperfectness. Thus, the fear of the imperfect number; 13.
Friday is believed to be an unlucky day in many cultures. Thus, friday the 13th is a very, very unlucky day.