So today is Friday the 13th. And if you're thinking, "big deal," you're right. It is a big deal. Such a big deal that nearly $1 billion worth of commerce won't happen, because people are afraid to fly, travel, and shop.

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

Actually, I'd bet you are. Maybe you're not a paraskevidekatriaphobe. That's the fancy Greek term for people who fear Friday the 13th. Paraskevi is the Greek word for Friday, and dekatria is how Greeks say 13. Phobe, as you might have guessed, relates to phobia. But I'll bet you do things every single day because--know it or not--you're a little bit superstitious.

When you yawned this morning (after getting up on the right side of the bed and starting your day off on the right foot), 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.

And the same goes for sneezing. You may say "Gesundheit!" or "Bless you!" to be a nice person.

But what you're actually doing is carrying on a tradition apparently started by the Roman emperor Tiberius. According to one legend, Tiberius would ride in his chariot blessing anyone who sneezed, because a deadly disease at the time, in its early phases, caused sneezing. The more blessings were passed around, the less likely people were to die--or so the thinking went. This same thinking continued in Europe in the Middle Ages, when people were dying of bubonic plague.

Saying "Bless you!" isn't so silly, of course. Combine it with the cover-the-mouth-and-protect-your-soul rule, and you are much less likely to spread nasty germs.

But Friday the 13th is special. It combines Friday--considered an unlucky day by some--with the number 13, which has long been considered bad news.

Maybe you never thought of Friday as unlucky. Friday's often payday for us working stiffs. It's the start of the weekend. There's even that "TGIF" restaurant that serves a pretty tasty fried-cheese appetizer (if you want to tempt fate and clog your arteries). 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. Talk about a snack that's spelled eternal heartburn, even in Disney films. What fruit knocked out Snow White? Right. The apple.

(The really amusing thing is that the old adage, "an apple a day keeps the doctor away," has actually been shown to be true.)

And 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. There, if you get a thirteen, you win.

About 90 percent of Otis 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.

And the fear is historic: A Norse myth has it that a great dinner party with 12 guests was ruined when a 13th crashed the event and killed the god of joy and gladness. There were also 13 guests at the Last Supper of Jesus Christ, who was crucified on a Friday. Finally, a correctly tied hangman's noose has 13 knots, and executions used to happen on Fridays.

So, it's no wonder that we sometimes make a big deal out of Friday the 13th, even today, when we're steeped in science and technology, and know a great deal about how the world works.

For most people, Friday the 13th probably means taking minor precautions. Like avoiding black cats. Since I have a black cat, I can say that's probably a good idea on most days of the year. (Sorry, Spot. But you're a bad kitty.)

If you want to get into the fun of it, though, Dossey has some wacky precautions you can take to stay lucky this Friday the 13th.

• Stand on your head and swallow a chunk of beef gristle
• Take a holey sock to the top of a skyscraper or a mountain
• Walk around the block with a mouthful of water--and be careful not to swallow it
• Tie a sack of peony seeds around your neck (not too tight, though)

If you don't want to get in on the fun of it, well, I'd say you're missing out. You might even be kidding yourself about how rational you really are.

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.

But I walked down the aisle myself with blue-painted toenails. So far, it's been a great marriage. Knock on wood.