Triskaidekaphobia (from the Ancient Greek tris meaning "3", kai meaning "and", deka meaning "10", and phobos meaning "fear" or "morbid fear") is fear of the number 13 and avoidance to use it. It causes the fear of Friday the 13th, called paraskevidekatriaphobia or friggatriskaidekaphobia.
The fear goes back to the Babylonian times. The fear magnified when Judas was the 13th person to sit at the Last Supper with Jesus Christ and 11 other people, and then ultimately hung himself in frustration.
Sufferers would avoid using things that designate 13, like going to channel 13, playing billiards without 13-ball, going to the 13th floor (though many buildings skip this designation), and even skipping question 13 in schoolwork. Some triskaidekaphobia sufferers who turned 13 years old don't want to be called a 13-year-old, instead they may continue to be called a 12-year-old after the 13th birthday, then skip to 14.