Fear of the number 13 is known as “triskaidekaphobia.”
Why is it considered unlucky? No one knows. Perhaps it is because Jesus was betrayed by the “13th” apostle. Maybe it is because we have 12 months in a year and 12 inches in a ruler, so the 13th month, or 13th inch is just odd.
Some say it is part of Norse legend, that 12 gods were invited to dinner, and the uninvited Loki showed up as number 13 and caused one god to shoot another with a mistletoe tipped arrow.
And perhaps it is reinforced by Apollo 13 which was nearly tragic.
My own theory is that there is an organization that goes around and causes people to have accidents on the 13th of each month. I’ve been recruited, so watch out!