Triskaidekaphobia refers to the fear of the number 13. It's one more than the dozen which leaves one unlucky one out if you divide something in groups of two, three, four, or six. It's
also said that there were 13 people in the Last Supper. Friday the 13th is considered especially unlucky by many, while in some cultures, in the Spanish-speaking world, for example, it's Tuesday the 13th that is believed to be unlucky.