I blame King Phillip. And nominate him as the patron ghost of root canals. Along with Popes.

Damn kings and popes - messing up perfectly good fridays!

http://en.wikipedia.org/wiki/Friday_the_13th#History

Another theory about the origin of this superstition traces to the arrest of the legendary Knights Templar.

The Knights Templar were a monastic military order founded in Jerusalem, in the year 1118. Their original mission was to guide and protect Christian pilgrims along the path from Europe to Jerusalem during the Crusades. Through this mission, the Templars developed a banking system to protect the finances of the traveling pilgrims, eventually expanding this banking system throughout their holdings in Europe. Over time, France's Philip IV amassed a substantial debt to the Knights Templar, due to their years of service to the crown. He had nearly depleted his money, from France's ongoing battles with England. In the Knights' rise to power, King Phillip became envious of them, so he set his sights on their famed fortunes. Philip devised a plan to arrest all the Knights in a single day, and charge them with crimes so heinous that no person or group would dare come to their defense. The charges against them were religious in nature, and backed by the papacy of the Vatican and Pope Clement V. His plan was swift and carefully crafted, so as not to alert the Templars in advance.

Knights Templar and Philip IV
King Phillip's orders were sent a month in advance to the King's Men and other Bailiffs, with instructions not to open the orders until dawn of Friday, October 13, 1307. The charges against the Templars were of the highest accusations of heresy: that the Knights Templar had asked members to spit on the cross and to step on it, to deny Christ, to perform homosexual acts, and so on. The king's orders were to engage and arrest every Templar in France. All Templar outposts, homes, wineries, mills, and castles were to be taken in the name of the King of France and Pope Clement V. The nationwide arrest was widely successful, with medieval torture tactics used to obtain confessions from the Knights. This act against the Templar Order is now viewed as one of the most unlucky days in History – Friday the 13th.