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. The superstition around Friday the 13th is deeply embedded in Western culture, and it is difficult to pinpoint the precise origins of it. However, both Friday and the number 13 have been regarded as unlucky in certain cultures throughout history.

There are many theories about why Friday the 13th is considered unlucky. For example, in the Bible, Judas, who is said to have betrayed Jesus, was the 13th guest at the Last Supper, and many unfortunate things happened on Fridays. In Norse mythology, Loki, the god of mischief, gate-crashed a banquet in Valhalla, bringing the number of gods in attendance to 13.

Despite these origins, the Friday the 13th superstition dates back only to the Middle Ages. The real Friday the 13th hysteria started in the 20th century, and many date this back to Thomas Lawsons book "Friday, the Thirteenth," which is about a stockbroker who chooses this day to deliberately crash the stock market. Later in the 1980s, the popularity of the "Friday the 13th" movie franchise added to the cultural phenomenon.

It is interesting to note that Friday the 13th is not considered unlucky in all cultures. In Greece and Spanish-speaking countries, it is Tuesday the 13th that is considered a day of bad luck, while in Italy, it is Friday the 17th that is met with fear.

In conclusion, Friday the 13th is considered an unlucky day in Western superstition, and it occurs when the 13th day of the month in the Gregorian calendar falls on a Friday. The origins of this superstition are difficult to pinpoint, but both Friday and the number 13 have been regarded as unlucky in certain cultures throughout history.