Friday the 13th will occur every year at least once, and at most three times a year. For any month, if the 1st is a Sunday, then that month holds a Friday the 13th.
Wikipedia has a few possible origin stories for the phobia of Friday the 13th. The movie Da Vinci Code made the following origin popular, but this may also just be a modern recasting of past events:
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 their rise to power, King Phillip became envious of the Knights Templar, 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 devised, so as not to alert the Templars in advance.
King Phillip’s orders were sent a month in advance to the King’s Men and other Bailiffs. They were not to be opened till dawn on Friday, October 13, 1307. The charges against the Templars were of the highest accusations of heresy: that the Knights Templar asked members to spit on the cross and 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. This nationwide arrest was widely successful, and medieval torture tactics were used to obtain confessions from the Knights. This single act against the Templar Order is now viewed as one of the most unlucky days in History – Friday the 13th. King Phillip attempted to further disgrace the Templars in a public manner. At a large event in front of the Notre Dame Cathedral, he would have Templar Grand Master Jacques De Molay publicly admit guilt of heresy. Instead, the defeated grandmaster took to his forum and apologized to the people and Templar Knights for his weakness and for signing forced confessions. He then rescinded his original confession and testified to the public that he, his men, and all Templar Knights were innocent, despite their forced confessions. An embarrassed King Phillip was enraged by the old man’s actions and had him burned at the stake along with his second-in-command. De Molay’s dying last words were to curse King Phillip and Pope Clement V, claiming that by the year’s end they both would meet their demise. Both men died that year, thus adding to the superstition of the Friday the 13th, and to the notion of the Templars’ powers.
Things are not quite what they seem.