Here’s the boring wikipedia explanation of Friday the 13th. It occurs when the thirteenth day of a month falls on a Friday, which superstition holds to be a day of bad luck. In the Gregorian calendar, this day occurs at least once, but at most three times a year. Any month’s 13th day will fall on a Friday if the month starts on a Sunday. The fear of Friday the 13th is called friggatriskaidekaphobia (Frigga being the name of the Norse goddess for whom “Friday” is named and triskaidekaphobia meaning fear of the number thirteen).