Paraskevidekatriaphobics is quite a mouthful. But what it means is relatively simple. An irrational, terrible fear of Friday the 13th. It's claimed by some to be the most widespread superstition in the U.S., even to the extent that studies have been done to try and confirm the fear. One in the UK showed a supposed higher risk of hospitalization due to car accidents on Friday the 13th as compared to other Fridays.
There are a lot of reasons people have for why the number 13 is unlucky. The two most common being that there were 13 at The Last Supper and Judas, being the first to rise, was the first to die. Also that the raid on the Knights Templar was planned for October 13, 1307, a Friday the 13th.
However, despite studies and reasons like these, I have no fear of this day. One reason? It's my birthday! I also turned 13 on a Friday the 13th, and everything was extremely pleasant. So whether or not you have triskaidekaphobia (and if you do, here's some helpful hints to get over it), make sure to have a safe and wonderful day!