Friday the 13th - how did that day and date combination become foreboding? Many historians connect it to the Last Supper (13 men, a betrayal and the Good Friday crucifixion - can't get much worse). Another theory rests on the downfall of the Knights Templar (hundreds of religious knights executed in France on Friday, Oct. 13, 1307).
Superstitions are defined as a set of irrational beliefs in the supernatural that are based in the fear of the unknown, born from ignorance or exist due to contrary proof (dictionaries don't mince words). While some are sourced in paganism and witchcraft, many superstitions are drawn from Christianity, folklore and mythology. Some believe "knock on wood" (yup, my own previous post) comes from knocking on a wooden cross.
The number 13 itself has its own particular "bad juju". It's not just those with triskaidekaphobia who accept the fact that it's best to be without a floor numbered 13. Global differences abound; some Asian cultures fear the number four, and Italians don't like 17.
Pet a black cat, fold up your ladders when you're done, and have a good day!