Friday the 13th is a day that people associate with bad luck. The day strikes three times a year, those being Jan. 13, April 13 and July 13. The spooky part of those three days? They are each 13 weeks apart. So how did approximately 21 million people start to fear Friday the 13th?

According to Donald Dossey, a folklore historian and author of "Holiday Folklore, Phobias and Fun," fear over the day stems from "ancient, separate bad-luck associations with the number 13 and the day Friday." When the two combine they make a "super unlucky day."

Myth

One story shared by Dossey, according to National Geographic is a Norse myth. Twelve gods were said to be having a dinner party when a 13th guest arrived uninvited. The unwanted guest had managed to "arrange" for the blind god of darkness, Hoder, to shoot a mistletoe-tipped arrow at Balder the Beautiful, god of joy and gladness. According to Dossey, when Balder died, "the whole Earth got dark. The whole Earth mourned."

Thirteen being an unlucky number can also be traced back to the Last Supper. Because of 13 people in attendance, unlucky events followed suit.

12

The number 12 is supposed to be a "complete" number, which makes the next number, 13, suffer. Twelve is a complete number because there are 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus,12 labors of Hercules, 12 tribes of Israel and 12 apostles of Jesus, reports National Geographic. Thirteen is unlucky because it is "beyond completeness," says Thomas Fernsler, associate policy scientist in the Mathematics and Science Education Resource Center at the University of Delaware in Newark.

Jason

As if people weren't afraid enough of the day, Paramount Pictures made a horror movie in 1980 entitled, "Friday the 13th." The character Jason, who was born on Friday the 13th, exactss revenge on Friday the 13th by murdering a slew of campers. Numerous remakes and sequels have spawned from the first horrifying film.

Superstition

Friday the 13th and the number 13 in general have a great effect on most people. Here are some odd things that people won't do because it's unlucky:

-Most tall buildings don't have a 13th floor.

-According to the Stress Management Center and Phobia Institute, approximately $700 to $800 million is lost every Friday the 13th. People don't want to travel or purchase major items on the unlucky day.

-Some believe that if a child is born on Friday the 13th they will be unlucky for life.

-Time magazine reports a myth that if you cut your hair on Friday the 13th, someone in your family will die.

-Another myth that frightens people is if a funeral procession passed you on Friday the 13th you will be the next to die.

 How to keep away the bad luck

Just because the day is unlucky doesn't mean that you have to suffer. Yahoo News suggests that on Friday the 13th individuals should: stay away from pavement cracks, knock on wood, keep fingers crossed, avoid walking under ladders, invest in a horseshoe, carry a rabbit's foot, don't break a mirror, avoid black cats and don't get up on the wrong side of the bed.