Many people consider the number 13, unlucky. That is why commercial and residential buildings often don't have a designated '13th' floor. But when the day falls on a Friday, the fear factor increases substantially. The good news is that this is the only one we will encounter in 2014. The bad? For the first time in 14 years, it coincides with another bad omen - a full moon!

Most of you are probably shrugging this off and just thankful that the weekend is here! However, there are over 20 million Americans and many more worldwide, that suffer from friggatriskaidekaphobia. Derived from the words 'frigg', the Norse goddess after whom Friday is named and 'triskaidekaphobia', the irrational fear of the number '13', these people experience symptoms that range from mild anxiety to a constant fear that something bad is going to happen, to even full-blown panic attacks. Since many that suffer from this phobia work in high injury professions like construction and ship building, their nervousness often results in a self-fulfilling prophecy. This in turn, makes them even more fearful of the day.

Though nobody knows for sure why the combination of this date and day are so feared, there are numerous theories. According to experts, it all starts with the irrational fear of the number 13 that Triskaidekaphobiacs have, regardless of what day it falls on. Thomas Fernsler believes the apprehension is caused by the position of 13, which follows the nicely rounded 'safe' 12. The Associate Policy Scientist from University of Delaware says that people often perceive 12 to be a complete number - there are 12 months in a year, 12 zodiac signs, 12 tribes of Israel, 12 apostles of Jesus . . . so on and so forth. Therefore, the number right after that seems incomplete and therefore, deemed unlucky.

Folklore historian Donald Dossey traces the fear all the way back to ancient times, when the Vikings inhabited Scandinavia.Twelve gods were having a dinner party at Valhalla, the Viking's mythological heaven. Suddenly, an uninvited '13th' guest, the mischievous god Loki walked in and instigated Hoder, the blind god of darkness to kill Balder, the god of joy and gladness.

As for Friday? That, happens to be rooted in ancient mythology and religion - Per the bible, Jesus was crucified on a Friday. Though the date is not believed to be the 13th, it was following the Last Supper, which was attended by 13 men that included Judas Iscariot, the apostle that betrayed Jesus.

Other biblical scholars maintain that this was the day when Eve tempted Adam with the forbidden fruit - Again, the date is a little hazy. The one bad event that is believed to have happened on Friday the 13th, was the slaying of Abel by his brother Cain (The sons of Adam and Eve).

While the fear of a full moon is not as acute, there are many that believe that it pulls water in the body, similar to how it does tides in the ocean. They think that this phenomenon causes otherwise normal people to behave oddly and makes them prone to accidents.

So how bad is the fear? Every Friday the 13th, experts estimate that businesses lose between $800-$900mm USD. That's because millions of people avoid traveling and shopping. Some even call in sick and spend the day at home. The full moon added to the mix is bound to make things even worse! In case you are wondering, the next time the two will coincide will be in 2049 - So enjoy this special day or . . . not!

Resources: foxnews.com,npr.com,news.yahoo.com