Paraskevidekatriaphobia , noun;
1. Fear of Friday the 13th
According to the Oxford English Dictionary, triskaidekaphobia means "fear of the number 13" and comes from Greek tris + kai + deka + phobia ("3" + "and" + "10" + "morbid fear") and is first attested in 1911. Paraskevidekatriaphobia is not included in the OED, but from other sources (...cough*Wikipedia*cough...) it comes from adding the Greek word Paraskevi ("Friday") to dekatreis ("13") and phobia to get "morbid fear of Friday the 13th.
Superstition surrounding Friday the 13th appears to be a modern invention, with little to no references to it before the late 19th century. The origin is not really known, but there are plenty of superstitions about the number 13 - mostly because 12 seems to be such a good number that 13 gets the shaft. There are 12 months in a year, 12 hours on a clock, 12 gods of Olympus, 12 tribes of Israel, 12 Apostles of Jesus, etc. Friday is also unlucky, and has been considered so since at least the 14th century. We've carried this into modern times by calling stock market crashes and other disasters Black Friday if they happen to fall on the last day of the working week. Black Friday is also the biggest shopping day of the year, right after Thanksgiving, but some might call that a particular kind of disaster as well.