Paraskevidekatriaphobia, noun, fear of Friday the 13th
We can barely pronounce it either but as far as the word goes, Macmillan tells us that it was coined in the 90s by an American psychotherapist named Donald Dossey, and “is based on the Greek words paraskevi (‘Friday’) and dekatria (‘thirteen’) with -phobia as a suffix to indicate ‘fear’.”
A related term is triskaidekaphobia, which comes from the early 20th century, and is from the Greek ‘tris’ (three) and ‘deka’ (ten). As Macmillan notes, “This word forms the basis of a lexical variant friggatriskaidekaphobia, also meaning ‘fear of Friday 13th’. The prefix frigga is based on the name of an ancient Scandinavian goddess who was associated with witchcraft and Friday (the witches’ sabbath).”
While no one knows for sure how or why we fear Friday the 13th, About.com gives it a go with theories that range from our early selves (“Primitive man had only his 10 fingers and two feet to represent units, this explanation goes, so he could count no higher than 12. What lay beyond that — 13 — was an impenetrable mystery to our prehistoric forebears, hence an object of superstition.”); to the foundation of patriarchal society (“Thirteen had been revered in prehistoric goddess-worshiping cultures, we are told, because it corresponded to the number of lunar (menstrual) cycles in a year (13 x 28 = 364 days”); to Biblical ideas about the number (“[T]he Bible tells us there were exactly 13 present at the Last Supper. One of the dinner guests — er, disciples — betrayed Jesus Christ, setting the stage for the Crucifixion. Did I mention the Crucifixion took place on a Friday?”).
Among other theories, Wikipedia chimes in with this one:
In numerology, the number twelve is considered the number of completeness, as reflected in the twelve months of the year, twelve hours of the clock, twelve gods of Olympus, twelve tribes of Israel, twelve Apostles of Jesus, the 12 successors of Muhammad in Shia Islam, etc., whereas the number thirteen was considered irregular, transgressing this completeness. There is also a superstition, thought by some to derive from the Last Supper or a Norse myth, that having thirteen people seated at a table will result in the death of one of the diners.
Finally, here’s a wonderful urban legend that we wish were true. It runs like so:
Sometime in the 19th century, the [British] Royal Navy attempted to finally dispel the old superstition among sailors that beginning a voyage on a Friday was certain to bring bad luck. To demonstrate the falseness of this belief, they decided to commission a ship named HMS Friday. Her keel was laid on a Friday, she was launched on a Friday, and she set sail on her maiden voyage on a Friday, under the command of a Captain James Friday. She was never seen or heard from again.