Friday the 13th has had the reputation of being an unlucky day since the mid 19th-Century. However, the notion of lucky and unlucky days is ancient. Calendars from Egypt dating back as far back as 2040-1750 BCE have been found marked with such propitious and ill-fated days.
Friday (which takes its name from the Norse goddess Frigg) has long been consider an inauspicious day in Western culture. Sailors were known not to ship out on a Friday, travellers to stay home, and businessmen not to conduct business. Friday was also thought to be the day on which witch’s held their Sabbaths. Christian tradition also ascribed Friday as the day on which the worse events in their mythology took place; Adam and Eve were expelled from Eden on a Friday, Cain killed Able on a Friday, God flooded the world on a Friday, and Jesus Christ was crucified on a Friday.
Even older than the fear of Friday, however, is the fear of the number thirteen, the technical term for which is Triskaidekaphobia. Triskaidekaphobia is so prevalent amongst people even today that we are often told about how businesses and hotels will purposefully “omit” a thirteenth floor, jumping from 12 to 14. The oldest known example of this practice can be found in the Babylonian Code of Hammurabi (c. 1800 BCE) – the earliest known set of written laws – which omits law thirteen.
Other examples of thirteen being an ill-omen come from Norse mythology where the trickster Loki’s presence at a banquet makes thirteen guests and signals the beginning of the events that will lead to the death of Baldr, the much loved god of light, and eventually doomsday called Ragnarok. Likewise in Christian tradition Jesus’ last supper hosts thirteen guests (twelve apostles and Jesus) one of whom, Judas, will betray Christ.
Considering the mythical history both Friday and the number thirteen have its not at all surprising that the presence of the two together should signal certain doom for the superstitious.
However, it should be noted that not all cultures fear Friday or the number thirteen. In both Muslim and Jewish tradition Friday either is or marks the beginning of the Sabbath while in the religion of Sikhism thirteen is seen as holy and thus very lucky.
Source: Friday the 13th (2009) by Nathaniel Lachenmeyer at Fortean Times.com and Fertility Goddesses, Groundhog Bellies & Coca-Cola: Modern Holidays (2006) by Gabriella Kalapos.