The superstition surrounding Friday the 13th likely combines two older beliefs: that the number 13 and Fridays are each individually unlucky. The number 13 has been considered unlucky in various cultures and historical periods, partly because it follows the "complete" number 12 (e.g., 12 months in a year, 12 zodiac signs, 12 apostles). Judas, the apostle who betrayed Jesus, is said to have been the 13th to sit at the Last Supper.
These beliefs merged over time, although the exact origin of Friday the 13th as a particularly unlucky day is not well-documented. Some historians trace it back to the early 20th century, when the first references in popular literature appeared. Others point to older traditions and folklore that combine these elements of bad luck.
Regardless of its origins, the fear of Friday the 13th even has its own name - paraskevidekatriaphobia - and has been the subject of numerous movies, books, and articles. Although not all cultures consider the day or the number unlucky, there are examples of other superstitions that are said to be very unlucky:
Breaking a Mirror: Breaking a mirror is said to bring seven years of bad luck, possibly because mirrors were once expensive and considered magical.
Walking Under a Ladder: This is considered bad luck in many countries, often interpreted as inviting the supernatural or disrupting the Holy Trinity.
Black Cat Crossing Your Path: In some cultures this is considered a bad omen, although in others black cats signify good luck.
Opening an Umbrella Indoors: Common in Western superstitions, it's considered to invite bad luck as it supposedly offends the spirits residing in the home.
The Evil Eye: In Mediterranean and Middle Eastern cultures, the "evil eye" is thought to cause misfortune or injury. Protective amulets are often used to ward off this bad luck.
Number 4: In Chinese, Japanese, and Korean cultures, the number four is avoided because it sounds like the word for "death."
Spilling Salt: In some cultures, spilling salt is an omen of bad luck and throwing a pinch over your left shoulder is thought to counteract it.
Tipping a Salt Shaker: In Russian culture, tipping over a salt shaker is considered to bring bad luck and even foreshadow a future conflict.
Cracks in the pavement: The saying "step on a crack, break your mother's back" is common in the United States, although the superstition itself is largely considered to be a children's rhyme.
These are just a few examples, and many have variations and specific contexts in which they are believed to hold sway.