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 fear of 13 dates back centuries and many believe it originates from the Code of Hammurabi which reportedly left out a 13th law from its list of legal rules. In Christian tradition, fear of Friday the 13th stems from the day of the Crucifixion (Friday) and the number at the . Despite these origins, the Friday the 13th superstition dates back only to the Middle Ages.

While Friday the 13th may feel like a rare phenomenon, our Gregorian calendar means that the 13th of any month is slightly more likely to fall on a Friday than any other day of the week. It is not, however, a universal superstition: 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.

The origins of Friday the 13th as an unlucky day are difficult to pinpoint, but both Friday and the number 13 have been regarded as unlucky in certain cultures throughout history. In pagan times, for instance, Friday was believed to have a unique association with the divine feminine. In other cultures, the number 13 is considered lucky. For example, in Italy, the number 13 is associated with good luck and prosperity.

Despite the superstition surrounding Friday the 13th, there is no scientific evidence to suggest that it is an unlucky day. Many people believe that bad things are more likely to happen on Friday the 13th, but this is likely due to confirmation bias.