Triskaidekaphobia is fear of the number 13. It is usually considered a superstition.
It has been linked to the fact there were 13 people at the last supper of Christ, but it probably originated in medieval times. It has also been linked to that fact that a lunisolar calendar must have 13 months in some years, while the solar Gregorian calendar and lunar Islamic calendar always have 12 months in a year.
Triskaidekaphobia may have also affected the Vikings - it is believed that Loki in the Norse pantheon was the 13th god. This was later Christianised into saying that Satan was the 13th angel.
The Mesopotamian Code of Hammurabi (ca. 1686 BC) omits 13 in its numbered list. This seems to indicate a superstition existed long before the Christian era.