It’s Friday the 13th. Big deal, right? Well, actually… it is such a big deal that nearly $1 billion worth of commerce won’t happen today, because people are afraid to fly, travel, and shop.

But you’re not one of those people, right?

Maybe you are. Many people do things every single day because, know it or not, they’re a little bit superstitious.

When you yawned this morning, did you carefully cover your mouth? Probably so. It’s what we do to be polite. But there’s more to it than this–people used to think that your soul would rush out of your body if you issued an unprotected yawn.

But Friday the 13th is special. Maybe you never thought of Friday as unlucky. Fridays are payday for most working stiffs. It’s the start of the weekend. There’s even that “TGIF” restaurant. Friday is also the Muslim Sabbath and is the day for religious gatherings.

But Friday also carries negative connotations. For starters, Eve is rumored to have given Adam the apple on a Friday, according to Donald Dossey, founder-director of the Phobia Center in Asheville, North Carolina. Jesus was supposedly crucified on a Friday. And on which day of the week are the most pink slips handed out? You guessed it.

The number 13 has been feared for a long time, too–except in Italy, where “Fare tredici!” or “To make thirteen!” is the slogan of the national lottery, where, if you get a thirteen, you win.

Would you have purchased or rented your house or condo, if it was number 13? Did you know that more than 80 percent of elevators don’t have a button for the 13th floor? The U.S. Navy won’t launch a ship on Friday the 13th. And, as a former flight attendant once told me, many people just won’t fly on the 13th, unless they’re headed to Las Vegas. Apparently, the unluckiest day has an opposite effect when you’re gambling.

We usually think superstitious people just don’t know any better, but that’s pretty snobby of us. Superstitions are so common that sometimes we don’t even recognize them for what they are. Is it really bad luck, for example, for the groom to see the bride before the wedding? Why do brides wear something old, something new, something borrowed and something blue? Superstitions, all of them.

Ten years ago this fall, I got married with something blue in my bride’s pocket. So far, it’s been a great marriage. Knock on wood.