It is a common held fact that the number 13 is considered an unlucky number in many countries. For example, the nationalist political leader, Charles Stewart Parnell, had an irrational fear of the number thirteen, the Mayan calendar ended on the 13th Baktun, and was believed to be the sign of the end in the 2012 phenomenon, and many people tended to avoid buying products that was labeled as thirteen.
In 1911, a term for the fear of the number 13 was coined by Isador Coriat in his Abnormal Psychology book.
It was referred to as "Triskaidekaphobia," and it entailed that those affected by this phobia tried to avoid anything that had the number thirteen in it. This fear also extended to Friday the 13, which is considered as the unluckiest day of the month.
There are many interpretations as to why people are generally afraid of this number. However, a common consensus among people has not been met yet. Despite this fact, there are some theories as to why people are unnerved because of this number. For example, many people find the number to be unlucky because Jesus Christ, on the night of his upcoming trial and execution, arranged for his Apostles to have one last supper with him before his untimely demise. There were twelve disciples with him by the time of the holy prophet's death, and Judas Iscariot ultimately served as the one to betray him to the authorities.
There is also the belief that this fear of the number thirteen had something to do with King Philip IV of France ordering the arrests of the Knights Templar on Friday 13, 1307 A.D. Many of the said religious organization of Christian knights were tortured and executed on the spot.
Whatever the reason is as to why people fear this number might as well be left a mystery. Rest assured, however, people will continue to fear this number despite not knowing why they fear it.