You've made it almost through Friday the 13th with nothing out of the ordinary happening so far -at least we hope so. But there is another one coming in December. Why is Friday the 13th so unlucky? Or rather, who do some people think it's unlucky? There's no hard-and-fast answer, but instead, a lot of possibilities.
For whatever reason, among many cultures, the number twelve emerged throughout history as a "complete" number: There are twelve months in a year, twelve signs of the zodiac, twelve Gods of Olympus, twelve sons of Odin, twelve labors of Hercules, twelve Jyotirlingas or Hindu shrines where Shiva is worshipped, twelve successors of Muhammad in Shia Islam, and twelve tribes of Israel. In Christianity, Jesus was betrayed by one of his twelve Apostles—Judas—who was the thirteenth guest to arrive for the Last Supper. Surpassing the number twelve ostensibly unbalances the ideal nature of things; because it is seen as irregular and disrespectful of a sense of perfection, the number thirteen bears the stigma of misfortune and bad luck we know today.
But that doesn't tell us anything about Friday, or how the superstition came to be so widespread. Read more on those questions at mental_floss. Link
(Image credit: Flickr user Jim Rafferty)