If you saw a black cat today, would you turn and run away?

Yes, it's Friday the 13th, and for those that are superstitious, it's the ultimate day of bad luck. 

But why do (some) folks get all bent out of shape about Friday the 13th? Some theorize that it's an amalgamation of two older superstitions: that 13 is an unlucky number and that Friday is an unlucky day.

The fear over 13 comes from the fact that 12 is considered the number of completeness, as reflected in the 12 months of the year, 12 hours of the clock, 12 gods of Olympus, 12 tribes of Israel and 12 Apostles of Jesus.

By contrast, 13 is considered irregular, breaking this completeness. Along these lines, some believe that having 13 people seated at a table will result in the death of one diner. (Go ahead...invite 12 friends over for dinner tonight and see what happens)

Friday has been considered an unlucky day at least since the 14th century's "The Canterbury Tales," and many others have regarded Friday as an unlucky day to undertake journeys or begin new projects. Black Friday has also been associated with stock market crashes and other disasters since the 1800s.

Put these two fears together, and you have "friggatriskaidekaphobia," or fear of Friday the 13th.

For what it's worth, Spanish-speaking countries and Greece treat Tuesday the 13th as unlucky. And Italians actually consider 13 lucky. Their day to avoid? Friday the 17th.