Today is Friday, the thirteenth, which superstition holds to be a day of good or bad luck. In the Gregorian calendar, this day occurs at least once, but at most three times a year. Any month’s 13th day will fall on a Friday if the month starts on a Sunday. There were three Friday the 13ths in 2009, in the months of February, March, and November. There won’t be another Friday the 13th until August of 2010; the next one after that is May 2011. The next year with three Friday the 13ths will be 2015.

The fear of Friday the 13th is called paraskevidekatriaphobia. According to some sources it’s the most widespread superstition in the United States today. Some people refuse to go to work on Friday the 13th; some won’t eat in restaurants; many wouldn’t think of setting a wedding on the date. via About.com

- Christian origins of the fear of Friday the 13th: The fear of Friday the 13th stems from two separate fears — the fear of the number 13 and the fear of Fridays. Both fears have deep roots in Western culture, most notably in Christian theology. Thirteen is significant to Christians because it is the number of people who were present at the Last Supper (Jesus and his 12 apostles). Judas, the apostle w­ho betrayed Jesus, was the 13th member of the party to arrive.