3th person, Judas, betrayed Jesus. Jesus was then crucified on what is now called Good Friday. Another hypothesis is that it originated when King Philip IV of France arrested many members of the Knights Templar on Friday October 13, 1307.
Regarding the bad luck of the number 13 specifically, there are many myths and fairy tales from various cultures that depict 13 as a bad luck number. One is the Norse myth of a dinner party in Valhalla in which Loki was the 13th guest who bought bad luck. This myth and other tales could have been adapting plot elements and this superstition that may have originated from the Last Supper. However, there seems to be superstition surrounding this number that predates Christianity. Zoroastrianism was an ancient religion predating even Judaism that believed that the 13 millennium would be bad luck. It is possible that there was some cultural association of bad luck with this number that led to many stories using 13 as a bad luck number. This may have led to books and movies using this same superstition in their plots. These books and movies made more people in modern day superstitious of this number.
Why did ancient cultures have a negative association with the number 13 though? We don’t know for sure but a good inference is that it was because 12 was so often thought of as a complete number and so 11 or 13 would seem incomplete and thus would have a negative connotation to it. So why isn’t Friday the 11th bad luck. Well maybe 13 was unlucky because that was the average time puberty hits. We know many ancient rituals and superstitions surrounded sexuality and Abrahamic religions often shunned sexuality. This combination may have made 13 associated with bad luck. This is plausible but then why Friday the 13th? Well we don’t know but it is certainly possible that 13 had superstitions surrounding it before Christianity but superstitious people learned about the Last Supper and Good Friday and made Friday the 13th superstitious.
There are other days of bad luck for other cultures though. 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.