The fear of the number 13 has been given a scientific name: “triskaidekaphobia”; and on analogy to this the fear of Friday the 13th is called paraskevidekatriaphobia, from the Greek words Paraskeví (Παρασκευή, meaning “Friday”), and dekatreís (δεκατρείς, meaning “thirteen”)

The superstition surrounding this day may have arisen in the Middle Ages, “originating from the story of Jesus’ last supper and crucifixion” in which there were 13 individuals present in the Upper Room on the 13th of Nisan Maundy Thursday, the night before his death on Good Friday.  While there is evidence of both Friday and the number 13 being considered unlucky, there is no record of the two items being referred to as especially unlucky in conjunction before the 19th century.

An early documented reference in English occurs in Henry Sutherland Edwards’ 1869 biography of Gioachino Rossini, who died on a Friday 13th:

He [Rossini] was surrounded to the last by admiring friends; and if it be true that, like so many Italians, he regarded Fridays as an unlucky day and thirteen as an unlucky number, it is remarkable that on Friday 13th of November he passed away.

Friday the 13th is considered an unlucky day in Western superstition. It occurs when the 13th day of the month in the Gregorian calendar falls on a Friday, which happens at least once every year but can occur up to three times in the same year. In 2017, it occurred twice, on January 13 and October 13. In 2018, it will also occur twice on April 13 and July 13. There will be two Friday the 13ths every year until 2020, where 2021 and 2022 will have just one occurrence, in August and May respectively. (Wikipedia)