I'm sorry, Some of those answers so far are vaguely close but not quite on the head.
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Varied Reputations
Both the number 13 and the day of the week Friday have had varied "reputations" throughout the centuries - sometimes considered to be holy and at other times considered to be unholy.
In the Western world, the superstition linking Friday the 13th with being an unlucky day appears to be primarily associated with the Knights of the Templar and its final Grand Master, Jacques Demolay, and the date of Friday, October the 13th, 1307 AD (Old Style - OS)
The History
In 1099 AD the all powerful (at that time) Roman Catholic Church - via the now infamous bloody religious wars of the Christian Crusades - had regained control over the holy city of Jerusalem. However, the lands surrounding Jerusalem were still in the hands of the Moslems.
A warrior order of the Roman Catholic Church was banded together (allegedly) for purposes of protecting Christian travelers on their way to the holy city of Jerusalem.
The warrior order was finally officially recognized by the Roman Catholic Church in 1118 AD, and they were at that time provided with a headquarters building located on Mount Moriah. This building was located on a site traditionally thought to be the former Temple of King Solomon. Thus, the warrior order became known as the Knights of the Temple, or the Knights Templar.
To make a very long (but incredibly interesting) story short, Jacques Demolay became the Grand Master of the Knights Templar in around 1293 AD During the two hundred years of their existence within the Roman Catholic Church, the Knights Templar had grown into a rather rich and powerful political force to be reckoned with.
Because of this, during Demolay's stay in office as Grand Master, the Templars had increasingly grown in disfavor with the current pope, Pope Clement V, and with the King of France, Phillip the Fair. They became very greedy regarding the Templars wealth.
The Betrayal
In 1307 AD the pope and the king arranged for the Templars to meet with them for a "friendly" convocation in Paris, France. Instead of being friendly, the Grand Master Jacques Demolay was arrested on Friday, October the 13th, 1307 AD (Old Style).
Seven years later, in 1314 AD, Jacques DeMolay was burned at the stake for the crime of church heresy. Legend continues on that before Demolay died - he predicted to the king and pope that he would meet them both in heaven within the next year. Fact is... the king and the pope did die in the year 1314 AD. It is not known whether or not that Jacques DeMolay actually met up with these two rascals in heaven."
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In 1919 an organization was formed with DeMolay as the name. The Order of DeMolay was created by a Mason, named Frank Sherman Land, in Kansas City Missouri. It grew rapidly over the last 90 years and spread to 7 other countries and having over 1000 chapters in the U.S. Today it is a fraternity that continues to spread. (and Missouri DeMolay has been #1 in the world for i think 4 years now :)) In DeMolay, this act of loyalty and martyrdom is common knowledge.
DeMolay alumni include Walt Disney, John Wayne, Walter Cronkite, football Hall-of-Famer Fran Tarkenton, legendary Nebraska football coach Tom Osborne, news anchor David Goodnow and many others. Each has spoken eloquently of the life-changing benefit gained from their involvement in DeMolay
there are plenty of other websites that tell the same(-ish) story, but they all are pretty close. And please, anyone who is reading this, take care not to consider DeMolay a cult. As a member of this fraternity, i have seen more people urn down an unbelievably amazing (and christian-based) oppotunity because they had heard from somewhere else that it was a cult.
If anyone is interested in more information regarding the Order of DeMolay, email me at aaronjrollins07@hotmail.com.
God Bless,
/Aaron