Paraskevidekatriaphobics, this is a bad year for you

It's a bad year for people who suffer from paraskevidekatriaphobia -- the fear of Friday the 13th.

Why? There are three this year, instead of the usual two. There was one in 2011.

That's not all. For the first time since 1984, those three Friday the 13ths -- today, April 13 and July 13 -- are exactly 13 weeks apart.

Three Friday the 13ths happen every few years. The last was in 2009, and the next is 2015. What's special about 2012 -- and what won't occur again until 2040 -- is that this is happening during a leap year, says Tom Fernsler, a University of Delaware math professor who sometimes goes by the name Dr. 13.

It's hard not to feel something about Friday the 13th, Fernsler says. "I find that 95% of people in the world are superstitious about something, and the other 5% are liars," he said.

The number 13 and Friday are recurring presences in mythological, spiritual and religious tradition. In Christianity, 13 people attended the Last Supper before Judas' betrayal and Jesus' death on a Friday. A Norse myth warns of dire consequences for dining in groups of 13. Friday the 13th was the date the medieval Knights Templar were imprisoned. And, though not as historic, rapper Tupac Shakur died on Friday, Sept. 13, 1996, after being gunned down Sept. 7.

For many pagans, 13 is a lucky number, because it corresponds with the number of full moons each year, says Ivo Dominguez Jr., owner of Bell, Book & Candle, a pagan and occult book and gift shop in Dover, Del.

While Friday the 13th can feel special, it isn't. In a 400-year period, the 13th falls on a Friday more than any other day: 688 times, Fernsler says.

Fernsler has made it a hobby to find dates and events associated with the number 13, as well as Friday the 13th. The day has hosted some famous births, such as Fidel Castro and Butch Cassidy.

Dominguez argues Friday the 13th fears are "self-fulfilling prophecy," adding there's a "strong power in people convincing themselves something unlucky will happen."