Triskaidekaphobia is the fear of what

Fear of the number 13
Triskaidekaphobia (from 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 is a superstition and related to the specific fear of the 13th person at the Last Supper being Judas, who betrayed Jesus Christ and ultimately hanged himself.

It is also a reason for the fear of Friday the 13th, called paraskevidekatriaphobia (from Παρασκευή Paraskevi, Greek for Friday) or friggatriskaidekaphobia (after Frigg, the Norse goddess after whom Friday is named in English).