13

13 is, depending on your culture, either a terribly "unlucky" and ominous number or a happily lucky good-tidings number. As a result, there are many superstitions about it. There's even a word for the fear of 13: Triskaidekaphobia. Try saying that thirteen times in a row (or, well, even once).

Origin
The origin of the superstition is unknown. Thirteen is an unlucky number in some but not all systems of numerology and number woo.

Tradition holds that Judas Iscariot was the thirteenth person to sit down at the Last Supper (Jesus Christ and his twelve disciples), after which Jesus would be betrayed. Loki was the thirteenth Norse god at the feast where he killed Baldr. The reasons why these (un)worthies were symbolically placed thirteenth, though, is lost in the mists of time.

Impacts of triskaidekaphobia

 * The unpopularity of 13 goes back a long way: the, the earliest known code of laws, has no thirteenth law but goes up to 65. Hammurabi's triskaidekaphobia is a myth! The laws were not numerated, and you can easily verify the versions perpetuating the myth have a law missing; e.g. Compare the scared-texts.com text with that at Wikisource).
 * In nineteenth-century Europe, it was commonly believed to be very bad luck to have a dinner party or other occasion with exactly thirteen people. Even FDR was afraid to have such a party.
 * Friday the 13th
 * There are buildings that don't have a designated thirteenth floor (floor numbers jump from 12 to 14).
 * In Ireland the Government felt obliged to alter car registration numbers in anticipation of faltering sales in 2013.

Friday the 13th
Friday the 13th is considered an unlucky day (but as is common with superstitions there are also some who consider the day lucky). Furthermore, it is commonly held to be a lucrative series of increasingly shite films the most unlucky date. The particular badness of Friday the 13th is attributed to 13 people believed to be present at the Last Supper and the day of Jesus' crucifixion (a Friday, according to the gospels).

While there is no evidence that bad things happen more often on the date, people who buy into the superstition may experience "bad luck" due to their fears affecting them, or they may feel they've experienced more bad luck on that day due to confirmation bias.

A person who is afraid of the date suffers from "friggatriskaidekaphobia" (from the Greek word for "thirteen" and "Frigga's Day", Frigga being a Scandanavian goddess worshipped on the sixth day of the week). Another term, "paraskavedekatriaphobia", was coined by Dr. Donald Dossey, who claimed that if you could pronounce the word, you would be cured of the irrational fear. And possibly have a sprained tongue. Yes, it's a fear so nice, they named it twice.

It is also the title of a horror movie franchise, starring a whacked-out guy in a 70s goalie mask running around killing everyone.

Origins
There is no written evidence for a Friday 13th superstition before the 18th century, so the origins are disputable. Woomeisters, believing that age equates to validity, have traced the superstition's origins via oral tradition to, amongst other things, Norse mythology.

A modern reinforcement of the superstition is from three fiction writers that trumpeted the fact that the Knights Templar were rounded up, prior to persecution, torture and execution, on Friday, October 13, 1307. This, of course, was jumped upon by Dan Brown in his "arse gravy" novel, The Da Vinci Code. The other two works of fiction were Born in Blood: The Lost Secrets of Freemasonry by John J. Robinson and The Accursed Kings (Les Rois Maudits) series by Maurice Druon.

Whichever, it's still bollocks.

People who hate 13

 * Triskaidekaphobics

People who like 13

 * Superstitious Italians
 * Mayans
 * Wiccans
 * The Skaven
 * People born on the 13th