Triskaidekaphobia is fear of the number13. Paraskavedekatriaphobia is fear of Friday the 13th.
Friday the 13th is considered to be a day of bad luck in many Western cultures.
A billion dollars in the economy is routinely lost on Friday the 13th due to people calling in sick or refusing to fly.
Hotels and elevators leave off the number 13. Henry Ford refused to do business on Friday the 13th.