Friday the 13th is said to be an unlucky day, but historians don't all agree where the belief comes from.
It appears both Friday and the number 13 have been associated with bad luck independently for some time.
According to National Geographic, the number 13 may have got its bad rap due to two less-than-gracious dinner guests, both the 13th guest at their respective dinners.
In Norse mythology, the mischievious god Loki, god of evil and turmoil, interrupted a dinner party of 12 other gods. The uninvited 13th guest arranged for Hoder, the blind god of darkness, to shoot the god of joy and gladdness with a mistletoe-tipped arrow.
There is also the biblical story of the last supper and Judas' betrayal of Jesus. Judas was the 13th guest at the supper.
Interestingly enough, Franklin D. Roosevelt would never host 13 guests at a meal, according to LiveScience.com, which also reports Roosevelt refused to travel on the 13th of any month.
The association of Friday and bad luck appeared in The Canterbury Tales in the late 14th century, and in Christian tradition, the crucifixion of Jesus, Eve's offering the apple to Adam, the beginning of the Great Flood and the confusion at the Tower of Babble all are said to have taken place on a Friday.
By the way, if you're one of the 21 million Americans who suffer from paraskavedekatriaphobia, the fear of Friday the 13th (as opposed to triskaidekaphobia, fear of the number 13), don't worry, there's only one more Friday the 13th this year. It will occur on Nov. 13.