If you fear Friday the 13th, then batten down the hatches. This week's unlucky day is the first of three this year.

The next Friday the 13th comes in March, followed by Nov. 13. Such a triple whammy comes around only every 11 years, said Thomas Fernsler, a math specialist at the University of Delaware who has studied the number 13 for more than 20 years. Here are 13 more facts about the infamous day.