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For most of you, this Friday, which happens to be October 13, will mean little more than the start of a weekend. However, don’t tell that to the 20 million people worldwide that suffer from paraskevidekatriaphobia. They are convinced that the combination of the day and number brings bad luck. While no one knows for sure how Friday the 13th began to be so feared, there are numerous theories.

Experts think it starts with the irrational fear of the number 13, a condition called triskaidekaphobia. Thomas Fernsler, an Associate Policy Scientist at the University of Delaware, says the number suffers from its position after 12, which numerologists regard as a complete number. After all, there are 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus, and so on and so forth. This, according to Fernsler, has resulted in the belief that 13 is incomplete and, therefore, unlucky.

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The number’s bad reputation is exacerbated by the fact that several unfortunate incidents have involved 13. Among the most prominent examples is NASA’s Apollo 13 mission. The seventh manned mission in the Apollo space program, launched at 13:13 on 4/11/70 (the sum of the digits total to 13), never landed on the moon because of an explosion in one of its oxygen tanks on – you guessed it – April 13! Though the crew returned safely back to Earth, for the superstitious, the mission was jinxed because of its close association with the number.

If you are among the non-believers that can “Shake It (the superstition) Off,” you are are in great company. Taylor Swift loves the number. The singer-songwriter says, "I was born on the 13th. I turned 13 on Friday the 13th. My first album, Taylor Swift, went gold in 13 weeks. My first #1 song had a 13-second intro. Every time I've won an award I've been seated in either the 13th seat, the 13th row, the 13th section or row M, which is the 13th letter. Basically, whenever a 13 comes up in my life it's a good thing."

The Last Supper (Photo Credit: Leonardo da Vinci [Public domain], via Wikimedia Commons

The fear of Friday appears to stem mainly from ancient mythology and religion. It was the day Jesus was crucified. Though the date was not the 13th, it happened after the Last Supper, which had 13 guests, including Judas Iscariot, the apostle who betrayed Jesus. Many biblical scholars also believe that Eve tempted Adam with the forbidden fruit on a Friday. And it was on Friday the 13th that Cain killed his brother Abel.

Thanks to these myths, whenever the date and day coincide, paraskevidekatriaphobics experience symptoms that range from mild anxiety to full-blown panic attacks. Since many work in injury-prone professions like construction and shipbuilding, their nervousness often results in a self-fulfilling prophecy. This, of course further adds to their anxiety.

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It is estimated that businesses lose more than $800 million every Friday the 13th because many people avoid traveling or shopping. Fortunately, this is the last time the dreaded combination will be encountered in 2017, and the next one will not be until April 2018.

Resources: wikipedia.org, businessinsider.com