I can understand how it could be confusing, if you're coming from a significantly different culture. Many of the curses appear to be related to significant dates on the Earth calendar-- often, though not always, holidays celebrated in or particular to the United States. There's a certain amount of superstition stating that when the thirteenth of a month falls on a Friday, as it does today, it will cause inordinate bad luck. It's related to a more general phenomenon of triskaidekaphobia-- essentially the belief that the number 13 is itself unlucky.