Ah now this is a bit of a mystery but one that has lasted down the ages. Because of many unconfirmed reasons Friday 13th is often considered bad luck.
For my take on things I think it dates back to the Knights Templars. On Friday 13 October 1307 the Knights Templars were ordered to death and died from a very gruesome torture.
It was in the 14th century, King Philip IV in league with Pope Clement V ordered all the Templars to be rounded up and executed. Hence marking the perceived Friday 13th bad luck.
They were accused of numerous crimes including treason and heresy which explains why both the King and Pope were responsible for their deaths. After they lost the Holy Land their influence waned.
So, you see there is no sorcery only a bit of unfortunate history which has lasted to this very day.