I don’t generally believe in superstitions, but no one can be ignorant of the old superstition associated with the number 13.
As the number 12 is considered to be a “complete” or “divine” number in religion (12 Labors of Herakles, 12 months in a year, 12 Olympian gods, 12 zodiac signs, etc.) the number 13 is considered to be “incomplete” and, therefore, bad things are associated with it.
Loki, the Norse god of tricks, introduced death and evil to the world after he was the 13th attendant at a dinner party in Valhalla, according to Norse legend.