Friday the Thirteenth is considered unlucky due to two main sources: Christianity and Paganism. Fortunately, this is the only Friday the Thirteenth we will have this year. The Christian part comes from the thirteen attendees at the Last Supper and Christ being crucified on a Friday. Thirteen is considered an unlucky number because it is one beyond twelve, the latter thought to be a number of completeness and perfection. Thirteen is also problematic because of the way many societies count months according to the lunar-solar calendar. When drawing up a calendar of twelve months, there's always some time left over, a hidden thirteenth 'month' in the year that we don't know what to do with an don't acknowledge.
Wiki: "Strikingly similar folkloric aspects of the number 13 have been noted in various cultures around the world: one theory is that this is due to the cultures employing lunar-solar calendars (there are approximately 12.41 lunations per solar year, and hence 12 "true months" plus a smaller, and often portentous, thirteenth month). This can be witnessed, for example, in the "Twelve Days of Christmas" of Western European tradition."
Wiki quotes O. J. Ivey on the pagan source for fears of Friday the Thirteenth: "The actual origin of the superstition ... appears also to be a tale in Norse mythology. Friday is named for Frigga [or Frigg], the free-spirited goddess of love and fertility. When Norse and Germanic tribes converted to Christianity, Frigga was banished in shame to a mountaintop and labeled a witch. It was believed that every Friday, the spiteful goddess convened a meeting with eleven other witches, plus the devil — a gathering of thirteen — and plotted ill turns of fate for the coming week. For many centuries in Scandinavia, Friday was known as 'Witches' Sabbath.'"
Perhaps there are reasons why Frigga has been associated with the number thirteen and bad luck other than monks banishing the goddess to mountaintops for witchery. Before that happened, the recognizable belt of stars in the constellation of Orion was associated by the Norsemen with Frigga's spinning wheel. The planet Venus was Friggjarstjarna, or 'Frigg's star.' Godchecker.com confirms that Frigga was Odin's wife and the queen of Asgard; she was the highest goddess of the Æsir pantheon. She also had a spinning wheel that spun clouds. So far so good.
Doppelgänger coming through: Rushing to meet Wotan, Fricka drives her rams furiously with her whip (1910). By Arthur Rackham.
Then things get tricky. Frigga has a gift of second sight, with some impairment. She also doesn't have a solid identity. Godchecker: She "knows everybody's destiny, but will never reveal it. She is also in charge of housekeeping on a big scale. She rides a broom and sweeps away clouds when they pile up. Sort of air traffic control for the Gods. Or could it be something to do with witchcraft?"
Frigga is surrounded by confusion. Because of the word designating her relationship to Odin had two meanings, it's hard to tell if she was his wife or daughter. There are also legends about her being unfaithful to her husband. She has the power to see fates and futures, but does not foresee her son's death by being pierced by mistletoe. If this plant was Frigga's blind spot, things look less good for her. The Dictionary of Superstitions (eds. Opie and Tatem) notes that mistletoe was held by Druids to be a plant of protection against injury from fire or water. By the 17th century, that idea had morphed into mistletoe being protection against witchcraft, sort of like garlic against vampires. This is why people decked their houses with mistletoe at Christmas-time - partly because the Druids thought it sacred, partly because it warded off evil for the coming year.
Another confusing thing about Frigga is that she has a doppelgänger, a goddess named Freyja, highest goddess of the Vanir pantheon of oracles associated with fertility, wisdom and precognition. Scholars still argue if the two goddesses are the same goddess: "There are clearly many similarities between the two: both had flying cloaks of falcon feathers and engaged in shape-shifting, Frigg was married to Odin while Freyja was married to Óðr, both had special necklaces, both had a personification of the Earth as a parent, both were called upon for assistance in childbirth, etc." Perhaps these uncertainties around Frigga are not unlike the hidden thirteenth month in the year.
This has nothing to do with Friday the Thirteenth, but personally my favourite superstition in the Dictionary of Superstitions is the old wives' tale that to cure children of bed-wetting, you should feed them roast mice (!) I'm sure that would stop even Jason in his tracks.