1. Friday the 13th. Nobody knows for sure how it got started but one theory is that it has biblical origins and is tied to the number of guests at the Last Supper and Jesus' crucifixion on Good Friday.
2. Breaking a mirror. Ancient Romans believed that mirrors held a piece of your soul . . . and a separate myth said that our body "renews" itself every seven years. So those two things together helped create the "seven years of bad luck" thing.
3. A black cat crossing your path. Black cats got a bad rap in the Middle Ages when they were associated with witchcraft and demons. That demon thing snowballed into the idea that if a black cat crossed your path, they were blocking your connection to God and path to Heaven.
4. Walking under a ladder. Back in medieval times, ladders were associated with the gallows where people were hanged. So a person who walked under one might be facing their own death by hanging in the near future. There was also the idea that the area underneath was HAUNTED.
5. Opening umbrellas indoors. Again, nobody knows for sure but one theory is that the ancient Egyptians used umbrellas to protect themselves from the sun, and using them indoors was considered an insult to the sun god.
6. Spilling salt. There are a couple reasons why spilled salt is supposedly bad luck. One is that salt was once an expensive trading commodity, so spilling it was just plain wasteful. The other is that it was considered a magical substance in ancient times where it was used to perform rituals . . . so spilling it meant you were letting in the devil.
7. Stepping on a crack. This one is from European and early American folk tales, which said that the empty spaces in cracks on the ground were actually connections from earth to the spirit world . . . and messing with them would cause bad luck.
BONUS: And just because its super cheesy here's the trailer to the original Friday the 13th movie. Whut up 1980!