I have heard that it is bad luck if a black cat crosses my path. I am not exactly sure what “bad luck” is but it is irrelevant. First of all, it is none of my business to tell a cat where it can walk or what fur color it must have. Aside from that, how am I supposed to know where a cat may have walked before got there and took that path?
Another superstition is that if a day is the thirteenth day in a month and it happens to be a friday, then somehow the day is cursed or causes bad luck. The ideas of naming days or having the days numbered on a calendar are human inventions. I don’t care about them any more than the black cats do.
If someone were to avoid something because they knew that it would harm them, then it would be a rational fear based on knowledge. Superstition is based on ignorant fear. To avoid stepping on cracks on a sidewalk to avoid breaking your mother’s back makes no sense. What connection is there between someone’s back and a sidewalk? Knowledge of the irrelevance of things can a person free.