Don’t step on any cracks or break any mirrors today; it’s Friday the 13th.

While most people will brush off the supposedly unlucky day, it’s a superstition — called paraskevidekatriaphobia — that afflicts between 17 and 21 million people, according to a study by the Stress Management and Phobia Institute in North Carolina.

If you can make it through today, you won’t have to worry again until October, when the second and final Friday the 13th of 2017 rolls around. While no one knows for sure where the jinxed nature of the day originated, fear of the number 13 dates back to at least 1700 B.C., according to the History Channel.

One theory is the phobia was founded in the omittance of the number 13 in the Code of Hammurabi, a Babylonian set of laws most famous for the phrase “an eye for an eye.”

Some of the other possibilities for fear of the number 13 are rooted in Christianity. After all, Judas Iscariot, who betrayed Jesus Christ, the central figure in Christianity, was the 13th disciple to arrive at the Last Supper. And according to the Bible, Christ was crucified on a Friday.

There’s also the fact that the precursor to 13, its cousin 12, is considered in many traditions to be a “perfect,” or complete number. There are two 12-hour periods in a day, 12 disciples in Christianity, 12 gods of Olympus and 12 tribes of Israel.

It’s common for buildings to skip from the 12th to 14th floor, and in some hotels and hospitals, including Southeast Georgia Health System’s Brunswick campus, you won’t find a room No. 13.

The modern associations with Friday the 13th may be blamed on a 1907 novel by a Boston stockbroker named Thomas Lawson, according to Time magazine. The book, “Friday the Thirteenth,” tells the tale of a sinister businessman who plots to crash the stock market on — you guessed it — Friday the 13th. According to Time magazine, a wildly successful advertising campaign helped to book sell nearly 30,000 copies in the first week of its publication. Nearly a decade later, the book was turned into a feature-length silent film.

Brunswick-based psychologist Greg Cox said Thursday he hasn’t seen many clients suffering from fear of the number 13 — called “triskaidekaphobia” — but he notes like other phobias, it’s cyclical.

“The biggest thing with phobias is that they are self-reinforcing,” he said. “The classic quote from (President Franklin) Roosevelt, ‘The only thing we have to fear is fear itself,’ is exactly what a phobia lives on.”

So if you’re feeling a little anxious today, Cox recommends trying to relax.

“You can’t become not anxious, but you can become relaxed,” he said. “Learning relaxation takes 6 to 8 weeks, and once you learn it, it’s yours for life.”

So if you’re feeling anxious this Friday the 13th, take a deep breath, put on some soothing music or take a long walk — just be careful you don’t step on any cracks.