Have you ever played with superstition in your stories? Whether Friday the 13th or some other superstition like walking under a ladder or crossing paths with a black cat, having a character who makes decisions based on some selection of superstitions can be rather entertaining. Have you seen the TV show Monk? That's not superstition, but the same sort of quirks must be remembered and always incorporated.
So where did the superstition about Friday the 13th come from? According to Wikipedia, it's relatively recent, but some cultures already considered Friday an unlucky day. Also 13 has it's own unlucky issues. Whereas 12 is considered the number of completeness - 12 months in a year - 12 hours on a clock - 12 gods of Olympus - 12 tribes of Israel - 12 Apostles of Jesus - 12 successors of Muhammad in Shia Islam - 12 signs of the Zodiac - to list a few. By comparison, 13 is considered irregular - transgressing the completeness of 12.
There is even one superstition used in the Harry Potter books - I forget which book specifically and I may be off on the details, but there was a point where Harry, Dumbledore and others were sitting around a table (on Halloween even, I think - not sure though) Someone joined them making 13 around the table and everyone got all quiet and stared around. The superstition derives from the Last Supper and a Norse myth, that having 13 people seated at a table will result in the death of one of the diners. The Harry Potter books is the first time I had heard that particular superstition, but the mention in Wikipedia reminded me of it.
It's important to note that Friday may not be the unlucky day in your corner of the world, or in the corner of the world you're writing about, just as 13 might not be the big bad number. It's important to do your research on such things.
So tell me, have you ever included a superstition in any of your stories? Have you read any? Tell me all about it.