The superstition that’s the combination of two separate superstitions (and a lot of magical thinking).
Superstitions are ideas that unrelated things are connected in some supernatural way. They’re frequently practices that are thought to bring about good or bad luck. Knocking on wood, walking under ladders, black cats, four leaf clovers, etc. are all classic western superstitions. Astrology and other fortune telling methods have a similar kind of magical thinking. The superstition of Friday the 13th is a combination of two separate superstitions: Fridays + the number 13.
The unlucky number
One of the earliest examples of 13 being an unlucky number comes from Norse mythology. Loki was the uninvited 13th god to attend a feast following the recent slaying of the god Baldr (who died because Loki had tricked the blind god Höðr into inadvertently killing him). Another unlucky dinner with 13 members was the Last Supper where Judas betrayed Jesus. This spurred a related number 13 superstition that dinners with 13 members are unlucky. The first person to rise from the table will be in store for ill fortune (akin to how Judas was the first to rise from the Last Supper and was met with ill fortune). However various workarounds include dividing the guests across two tables or just having everyone rise at the same time (which seem like pretty simple hacks).
Another reason 13 is considered unlucky is that it throws off the satisfying “completeness” of 12. There are 12 months in the year, 12 signs of the western zodiac, there were 12 gods of Olympus, the 12 labors of Hercules, the aforementioned 12 Nordic gods in attendance at the the meal following Baldr’s death, 12 tribes of Israel, the 12 apostles, etc.
Over time western culture’s fear of 13 has spread to a wide variety of outlets. Over 80% of tall buildings skip counting the 13th floor and instead call it the 14th floor. Hotels sometimes skip having 13th rooms, the 13th card in the major arcana of the tarot deck is the card for death, the 13th loaf of bread in a baker’s dozen was sometimes said to be for the Devil, cruise ships tend to skip having a 13th floor, in Florence some houses which should have an address of 13 are given 12 1/2, etc.
It’s Friday I’m in … trouble
The fear of Friday has mostly Judeo-Christian origins. Jesus was said to have been crucified on a Friday (or perhaps it was a Wednesday). The start of the Great Flood and the confusion at the Tower of Babel were both said to have taken place on a Friday. Eve supposedly tempted Adam with the forbidden fruit, and the resulting expulsion from the Garden of Eden, took place on a Friday. Further Cain killed Abel on a Friday. Unfortunately the Bible is silent on what calendar system was in use in the Garden of Eden or how they had Fridays at all.
Eating meat on a Friday is considered unlucky because it’s reminiscent of death and the crucifixion (but eating fish is apparently exempt from this bad luck somehow). Cutting your nails on a Friday is also considered unlucky for similar severing of the body related reasons. Over time Fridays became an inauspicious day to begin or finish things. Starting a voyage, starting a new job, finishing the production of an article of clothing, moving house, getting married, giving birth, etc. on a Friday have all been considered unlucky.
That said if you die on a Good Friday there’s a superstition that you go right to Heaven.
Two great tastes that taste great together
Bringing these two superstitions together seemed inevitable, the super-superstition of bad luck on Friday the 13th, but it’s relatively new. Friday the 13th is first mentioned as unlucky in the 19th century with the most famous example being the Friday the 13th, November 1868 death of Italian composer Gioachino Rossini.
Friday the 13th didn’t become more widely unlucky in pop culture until the 20th century. Most people credit the 1907 Thomas Lawson novel Friday, the Thirteenth, about a stockbroker who chooses that date to manipulate (and crash) the stock market, as the popularization of the Friday the 13th superstition.
But like all superstitions, an unlucky day & date combination is inconsistent and culturally specific. While English speaking countries think of Friday the 13th as unlucky, in Spain and Greece it’s Tuesday the 13th that’s supposed to be unlucky, but in Italy it’s Friday the 17th.
It’s all in your mind
Ultimately the idea that Fridays, or the number 13, or the combination of Friday the 13th, are in any way unlucky, is nonsense. If they were real they’d be universally held beliefs (not to mention some objective proof). Instead these three superstitions are mostly just inconsistent western ideas – people in the rest of the world are going about their lives unaware of the danger they’re supposedly in (and somehow surviving).
There is no evidence that Friday the 13th brings about an increase in unfortunate incidents or accidents. A 2011 study in the The American Journal of Emergency Medicine reviewed hospital emergency admission rates and found no significant difference between Friday the 13th to other days. In fact a 2008 Dutch study demonstrated the opposite may be true, that people are more cautious on Friday the 13th and as a result there are fewer road accidents.
The Friday the 13th movie franchise capitalizes on the superstition. Interestingly in Spanish speaking countries the movies are sometimes called Martes 13 (Tuesday the 13th) in keeping with the Spanish superstition around Tuesday the 13th, instead of Friday the 13th. Finally, the most important metal band of all time Black Sabbath released their eponymous debut album on Friday the 13th, February 1970.