Despite this year being 2013, Friday the 13th didn’t occur more than usual, happening twice (in September and December).

In 2012, there were three Fridays on the 13th of a month, each 13 weeks apart.

There are different theories on how Friday the 13th came to be, but the prevailing one is linked to The Last Supper. Judas, the apostle who betrayed Jesus, was the 13th person to arrive at dinner, making 13 an unlucky number on any day of the week. Add to that the fact that Jesus died on a Friday, and Friday the 13th gets its bad rap.

Although people are far less superstitious now than they were in the past, the Stress Management Center and Phobia Institute in North Carolina estimates that 17 to 21 million people in the US have a diagnosable phobia of Friday the 13th. The illness is called friggatriskaidekaphobia.

Part of the reason the number 13 is considered so treacherous, is simply because it’s right after 12. Numerologists consider 12 a “complete” number: 12 months complete a year, 12 signs complete the zodiac and 12 inches complete a foot.

Friday the 13th didn’t occur more than usual in 2013

The hangman’s noose traditionally contained 13 coils. The condemned also had to walk up 13 steps to the gallows.

You may have noticed that most buildings leave out the 13th floor, but did you know that planes often lack a 13th row and some hotels eliminate a Room 13? In Florence, the house between 12 and 14 is actually addressed as 12 and a half.

Some estimates suggest that the US economy loses up to $900 million every time there’s a Friday the 13^th – many people avoid business deals, or even work altogether, on the allegedly doomed date.

There’s no proof that natural disasters are more likely to occur on Friday the 13th, but Australia’s biggest wildfire, Florida’s costly Hurricane Charley and Kansas’s “Great Flood of 1951” all occurred on a Friday the 13th.

To prove Friday the 13th superstitions as nonsense, a group of affluent New Yorkers started a “Thirteen Club” in 1881. Thirteen people met every Friday the 13th and dined in Room 13. During the gathering, guests walked under ladders and through piles of spilled salt.

In some cases, unlucky days just depend on the language. Tuesday the 13th is unluckier than Friday the 13th in Spanish-speaking countries since “Martes” (“Tuesday” in Spanish) is derived from the name of the Roman god of war and destruction, Mars. The movie Friday the 13th was even renamed to Martes 13 in some parts of the world.