Triskadekaphobia is a superstitious fear of the number 13. The most popular cultural expression of this fear is the conception that Fridays occurring on the 13th day of a month (Friday the 13th) are considered bad luck. Of interest is that mathematically, Friday is the most common weekday in the Gregorian calendar on which the 13th can occur. | Triskadekaphobia is a superstitious fear of the number 13. The most popular cultural expression of this fear is the conception that Fridays occurring on the 13th day of a month (Friday the 13th) are considered bad luck. Of interest is that mathematically, Friday is the most common weekday in the Gregorian calendar on which the 13th can occur. |