A fear of Friday the 13th is said to be the most common superstition among Americans.

The phobia, known by the lengthy monikers friggatriskaidekaphobia and paraskevidekatriaphobia, is so debilitating for some people that there is even a treatment center set up to help people overcome it. The center holds periodic meetings and other events to help the fearful, including one today in Las Vegas, Nevada.

But for the uninitiated, the question remains: just what is the big deal about Friday the 13th? This year is a particularly good one in which to ask that question, as there are three Friday the 13ths on the 2012 calendar, each seperated by 13 weeks; the first was January 13. It's the first year in which such spacing has occurred since 1984.

Friday the 13th has been a spooky part of the collective culture for years, but in fact there appears to be no recorded evidence of a fear or superstition regarding Friday 13th before the 1800s. The earliest documented English reference to the day being unlucky was in an 1869 biography by Henry Sutherland Edwards.

Since then, the phobia has become a major phenomenon, spawning countless articles and the infamous Friday the 13th horror movies in which the fictional character of Jason brutally murders unlucky summer campgoers on the freaky date while wearing a white plastic hockey mask.

And it has certainly gripped the consciousness of many, from young kids to even presidents. In fact, President Franklin D. Roosevelt was so superstitious about the date that he refused to ride a train on Friday the 13th, Thomas Fernsler, a University of Delaware mathematician so versed in lore about the number that he is known as Dr. 13, told USA Today.

There are a number of theories about why Friday the 13th is considered such a scary, unlucky day, but one of the most prevalent is rooted in the deeply religious undertones to the fear of the date.

For instance, thirteen people are said to have attended the Last Supper before Jesus Christ died on a cross on a Friday, according to the Bible. The scene is told in John 13:21, in case the number of people dining wasn't creepy enough for you.

And Satanists have run with the theory that Friday the 13th is a fated day. The Church of Satan rings in Friday the 13th as a day when superstitions grip the pious: Celebrate Friday The 13th! Today is 'Friday the 13th,' and we Satanists enjoy the consternation a simple number can conjure amongst the superstitious, the church posted on its official website on a previous Friday the 13th. So, on this dreaded day, while the credulous cower we sally forth to indulge with gusto. Have a truly splendid day today and an even more enriching night, dear aficionados of the outré!

For normal people that's just creepy. For those with a fear of Friday the 13th, it's horrifying.

So as you go about your business today, be thankful if you're not wracked with friggatriskaidekaphobia and paraskevidekatriaphobia, as many of your fellow citizens aren't so lucky.