In Spanish speaking countries and Greece Tuesday the 13th is bad luck not Friday the 13th. In Italy, Friday the 17th is bad luck. In some ancient cultures 13 was a lucky number. Clearly there is no objective bad luck number that exists. Different cultures for different reasons associated bad luck with some numbers and some days. Our modern American culture’s superstition about Friday the 13 likely has roots in cultures that associated 13 with bad luck before Christianity for reasons we can only hypothesize about. The Last Supper and Good Friday probably influenced the bad luck day to be Friday but we also don’t know this.
We don’t know exactly why 13 or Friday the 13th were originally thought to be unlucky. The best we have are good inferences. However, our modern knowledge of cultures and science can assure us there is nothing to be superstitious about. If we are less superstitious we will probably see less negative events going on on this day also.