It’s Friday the 13th!

Why is this day considered unlucky?

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The superstition that stepping on a sidewalk crack is unlucky causes some people to carefully watch how they walk.

Beware of Friday the 13th! Many people think the day is unlucky. (Luckily, there’s only one Friday the 13th this year.) As legend has it, you should try not to break any mirrors, walk under a ladder, or even spill any salt. Why?

Folklore historians say it’s tough to pinpoint how the tradition came to be. Many believe it has religious origins. In the Christian faith, there were 13 guests at the table for the Last Supper, which was held the day before the Friday on which Jesus died. “You are reestablishing two things that were connected to that terrible event,” Dr. Phil Stevens, an associate professor of anthropology at the University at Buffalo, tells TIME. “The number 13 became a problematic number because of the number of people at that table during that fateful event,” Stevens says.

A Widespread Worry

Now, Friday the 13th has become a cultural wonder. Many Americans avoid the number 13 all together including hotel rooms and airplane seat assignments. Some event venues even charge couples less to get married on the day. There are many other theories of how the luckless day came to be. Simon Bronner, a professor of American studies and folklore at Pennsylvania State University, believes Friday the 13th is a way for people to pin their bad luck to a certain cause. But, there’s nothing special about the date itself. After all, the number 13 is even considered lucky in some countries, like Italy.

“It may be a case of religious folklore that rose to explain a belief. Psychologists treat [the fear of Friday the 13th] as real, but my sense is that…it’s something to blame. I think it was a constructed belief,” Bronner, 62, says.

Still, whether people choose to avoid black cats or tiptoe around sidewalk cracks on Friday the 13th, Stevens notes that it’s important not to mock other people’s beliefs about the day. “Sometimes they are deeply rooted cultural fears,” he says. “You can insult somebody by making fun of it. Some people have deep cultural taboos that you cannot change by denying them.”

Do you think Friday the 13th is unlucky? Has anything happened to you on Friday the 13th to make you think otherwise?