Friday the thirteenth
Friday the 13th is considered an unlucky day (but as is common with superstitions there are also some who consider the day lucky).  Friday the 13th superstitions are, in a few concise words, codswallop, piffle, tosh and flannel!
Basically, the superstition is an amalgam of Friday being unlucky - an idea dating back to the fourteenth century - and 13 being unlucky.
While there is no evidence that bad things happen more often on the date, people who buy into the superstition may experience "bad luck" due to their fears affecting them.
It is also the title of a horror movie franchise, starring a whacked-out guy in a 70s goalie mask running around killing everyone.
 Fancy words for a simple idea
A person who is afraid of the date suffers from "friggatriskaidekaphobia" (from the Greek word for "thirteen" and "Frigga's Day", Frigga being a Scandanavian goddess worshipped on the sixth day of the week). Another term, "paraskavedekatriaphobia", was coined by Dr. Donald Dossey, who claimed if you could pronounce the word, you would be cured of the irrational fear.  And possibly have a sprained tongue. Yes, it's a fear so good, they named it twice.
 Why not Wednesday the twenty-fourth?
The origin of the superstition is unknown, but it is most often attributed to the number of people traditionally believed to be present at the Last Supper (Jesus Christ and his twelve disciples) and the day of his crucifixion (a Friday, according to the gospels). In nineteenth-century Europe, it was commonly believed to be very bad luck to have a dinner party or other occasion with exactly thirteen people.
Some[Who?] attribute the superstition to the fact that the Knights Templar were rounded up, prior to persecution, torture and execution, on Friday, October 13, 1307. This, of course, was jumped upon by Dan Brown in his "arse gravy"  novel, The Da Vinci Code.
There is no written evidence for a Friday 13th superstition before the 18th century, so the origins are disputable. Woomeisters, believing that age equates to validity, have traced the superstition's origins via oral tradition to, amongst other things, Norse mythology.
Whichever, it's still bollocks.