Friday the 13th is considered to be the most unlucky day of the month. It occurs when the 13th day of the month falls upon a Friday on the Gregorian Calendar. This fear of Friday the 13th is rooted in several myths, and it also coincides with the fear of the number 13 as well. The following is several interpretations as to why people tend to fear this day of the month.
In Christian theology, the fear of this day of the month stems from two things. They are: the fear of Fridays, and the fear of the number 13. According to scriptures, prior to Jesus' arrest at the Mount of Olives, Jesus invited his disciples to fellowship with him before his day of wreckening. There were thirteen people that attended the Last Supper, and Judas Iscariot was ultimately the one to betray his master to the authorities. Many religious scholars also believed that Adam and Eve ate of the Forbidden Fruit on a Friday, and that the Great Flood began on a Friday as well. As a result of this belief, many Christians refused to perform projects on a Friday fearing that they were doomed to failure right from the very start. It was also believed that the fear of Friday the 13th was influenced by the early Catholic Church's fear of pagan religions.
When the Romans were writing the Gregorian Calendar, they set this day aside for Venus, the Roman goddess of love. This theory also states that later on when the Norseman were making advances on the the Gregorian Calendar, they named Friday after Frigg, a Norse goddess that symbolized love and sex. It's also believed that this day of the month is unlucky, because, apparently, Frigg would visit with a coven of witches, and it would proceed to bump the membership up to thirteen members of the cult. A similar Christian legend held that 13 is unlucky, because it symbolized twelve practioners of witchcraft coming into an alliance with the Devil.
Other beliefs state that Friday the 13th is an unlucky day of the month, because of the fact that the Norse god of mischief, Loki, murdered Balder on a Friday due to being jealous of Balder's fame as a hero to the other Norse gods. It's also believed that huge ships have also mysteriously disappeared without a trace on Friday the 13th.