In North America and Europe, a significant portion of the population behaves very strangely on Friday the 13th. They won't fly in airplanes, host a party, apply for a job, get married or even start a new project. Some people won't even come into work. In the United States, roughly 8 percent of the population is afraid of Friday the 13th, a condition known as paraskevidekatriaphobia. "Friday the 13th" as we know it has its roots in many traditions and cultures.

The superstition surrounding Friday the 13th is actually a combination of two separate fears -- the fear of the number 13, called triskaidekaphobia, and the fear of Fridays. The most familiar source of both these phobias is Christian theology. Thirteen is significant to Christians because it is the number of people who were present at the Last Supper (Jesus and his 12 apostles). Judas, the apostle who betrayed Jesus, was the 13th member of the party to arrive.

Christians have traditionally been wary of Fridays because Jesus was crucified on a Friday, It is therefore a day of penance for Christians. In addition to that, some theologians hold that Adam and Eve ate from the forbidden fruit on a Friday, and that the Great Flood began on a Friday. God tongue-tied the builders of the Tower of Babel on a Friday and the Temple of Solomon was destroyed on a Friday as well. In the past, many Christians would never begin any new project or trip on a Friday, for fear that the endeavor would be doomed from the start.