Friday the 13th in the calendar
Friday the 13th, also known as Black Friday, is considered an unlucky day in Western superstition. It occurs when the 13th day of the month in the Gregorian calendar falls on a Friday. The superstition surrounding this day may have arisen in the Middle Ages, "originating from the story of Jesus' last supper and crucifixion" in which there were 13 individuals present in the Upper Room on Maundy Thursday, the night before His death on Good Friday.[1][2] Other scholars claim that there is no written evidence for a "Friday the 13th" superstition before the 19th century, and the superstition only gained widespread distribution in the 20th century. The fear of the number 13 has been given a scientific name: "triskaidekaphobia"; and on analogy to this the fear of Friday the 13th is called paraskevidekatriaphobia, from the Greek words Paraskeví (Παρασκευή, meaning "Friday", and dekatreís (δεκατρείς, meaning "thirteen".[3]

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http://en.m.wikipedia.org/wiki/Friday_the_13th

There are three Friday the Thirteenths this year, and if that worries us, we might have to blame a group who were the sworn enemies of all superstition.
Whatever the reason, since time immemorial many have feared Fridays and thirteens.
But why did the two fears come together to create a superstition with a life of its own, marked throughout the English-speaking world?
Not for any mystical reasons, it seems. "From the astrological point of view there is no need to be concerned about Friday 13th ," says Robert Currey of Equinox Astrology.
Dates and days of the week used to be closely related to planetary movements and phases of the moon in a system dating back to the Babylonians, he says, but that's not the case any more.
Sonia Ducie is a numerology consultant who believes strongly in the innate energy of numbers - 13 is "all to do with transformation and change" she says, and she counts Friday as the fifth day, associated with movement.
"You can see how with those two numbers together, it could be very restless," she says, but adds: "It's down to us; the energy's neutral."

Thomas Lawson's book starts the Friday 13th artistic tradition that generated the multi-million dollar film franchise in the 1980s.
Why did the combined superstition arise, then?
In 1907 a book called Friday, the Thirteenth was published, by a stock promoter called Thomas Lawson. It was the inspiration for the Friday 13th mythology which culminated in the lurid film and TV franchises starting in the 1980s.
Lawson's book is a dark fable of Wall Street whose central character ruthlessly engineers booms and busts in the market to work revenge on his enemies, leaving misery and ruin in his wake.
In it he takes advantage of the jitters which the date Friday 13th could be relied on to produce in the market traders.
"Every man on the floor and in the Street as well has his eye on it. Friday, the 13th, would break the best bull market ever under way," one character says.
So in 1907 fear of that date was already an established superstition. A quarter century before, it was not.