Friday the 13th

Friday the 13th is an unlucky day in much of Western Europe, North America, and Australia. Many people avoid travel and avoid signing contracts on Friday the 13th. Floors in tall buildings often skip from 12 to 14. And while the superstition is believed to be fading, it nonetheless has deep roots in both Christian and pagan culture. 

Superstition is a credulous belief or notion, not based on reason, knowledge or experience. The word is often used pejoratively to refer to folk beliefs deemed irrational. This leads to some superstitions being called "old wives’tales". It is also commonly applied to beliefs and practices surrounding luck, prophecy and spiritual beings, particularly the irrational belief that future events can be influenced or foretold by specific unrelated prior events. 

The etymology is from the classical Latin superstitio, literally "a standing over", hence: "amazement, wonder, dread, especially of the divine or supernatural". The word is attested in the 1st century BC, notably in Cicero, Livy, Ovid, in the meaning of an unreasonable or excessive belief in fear or magic, especially foreign or fantastical ideas. By the 1st century AD, it came to refer to "religious awe, sanctity; a religious rite" more generally.

Source: Wikipedia