CHICAGO — Henry Ford would have hated 2009, and not just because it's been a tough year to sell cars.

Ford, as the story goes, refused to do business on Friday the 13th, and this week marks the third time this year that the 13th will fall on a Friday,

It's a day when people rearrange travel plans, delay surgery or just pull up the covers and stay in bed until Friday the 13th turns into Saturday the 14th, convinced that even stepping out of the house would cause bad luck to find them the way an anvil finds the head of Wile E. Coyote.

"They're afraid something tragic or ominous would happen," said Donald Dossey, a North Carolina behavioral scientist and author who said he named the fear, paraskavedekatriaphobia — proof that he does not suffer from hippopotomonstrosesquippedaliophobia, the fear of long words.

Some feel they're just being cautious the way Ford, Napoleon and President Franklin Roosevelt were said to have been.

Elizabeth Lampert, a consultant in Alamo, Calif., said she doesn't avoid everything on the 13th, but would "absolutely, absolutely" delay something like surgery.

"There are only a few Friday the 13ths, so why test fate?" Lampert said.

The phobia around the 13th is a cousin to triskaidekaphobia, the fear of the number 13. Even today, the Otis Elevator Company knows better than to include a button with a 13 on it in elevators all over the world, said spokesman Dilip Rangnekar. The supposedly unlucky number, triskaidekaphobes say, is the reason behind the explosion of Apollo 13, which took off at exactly 1:13 p.m. (1313 military time) on 4/11/70 (digits that add up to 13, naturally).

It's also the number that prompted FDR to alter his own travel plans on any day of the week that landed on the 13th.

"FDR would not depart on a (train) trip on the 13th," said Thomas Fernsler, a University of Delaware mathematician who has studied the number enough to earn the moniker "Dr. 13." He recounted a story that originated with FDR's personal secretary, Grace Tully, who said the former president would order the train to leave the station before midnight on the 12th or after midnight on the morning of the 14th.

In a final act, FDR died in 1945 on April 12. Thursday, April 12.

"He avoided traveling to the beyond on the 13th," joked Bob Clark, head archivist at the Franklin D. Roosevelt Presidential Library and Museum.

The origins of all this fear of the number 13 and Friday the 13th are open for debate.

Some say it has to do with a particular Friday the 13th in the 1300s, when some particularly unlucky knights were burned at the stake. Fernsler suspects it may have something to do with Jesus Christ, who was crucified on a Friday after a Last Supper attended by 13 people, one of whom was Judas Iscariot.

Dossey has his money on Norse mythology when Loki, referred to in the Encyclopaedia Britannica as a "cunning trickster", crashed a party of 12 gods at Valhalla.

"That's really when the number 13 became unlucky," he explained.

It is impossible to tell just how many people out there are changing their plans.

But one person who has made a living getting inside people's heads, The Amazing Kreskin, who bills himself as "the world's foremost mentalist" ,aid he's seen for himself how seriously people from all walks of life take Friday the 13th.

"There are many, many people in the business world who do not fly on Friday the 13th," said Kreskin, who legally changed his name from George Kresge Jr.

But in Chicago, for example, neither O'Hare International Airport nor United Airlines has noticed any drop in the number of people flying on Friday the 13th.

"It's an old wives' tale," said United spokeswoman Robin Urbanski in an e-mail.

The same goes for two of the biggest hospitals in the city — Northwestern Memorial Hospital and the University of Chicago Medical Center, where it appears people are just as likely go to go under the knife that day as any other, and aren't rushing to the doctor, either.

"When it's Friday the 13th, you wonder if it is going to be busier ... but people aren't coming into the ER panicking, saying 'I just realized it's Friday the 13th, give me Xanax or Valium,'" said Dr. Pedro Dago, a Northwestern psychiatrist.

Not only that, but if Wall Street is any indication, Friday the 13th may actually be a lucky day. The stock market, it turns out, tends to do better on Friday the 13th, rising by an average of .04 percent on each of the past 185 Friday the 13ths, according to the Bespoke Investment Group, a Harrison, N.Y.-based investment research firm. That's double the average .02 percent gain, but a little worse than other Fridays, which are generally good days for stocks.

And Friday the 13th might be just the thing to prompt people to go ahead with their plans.

Lampert, the same consultant who said she would not have surgery on that day, said she originally had reservations about agreeing to a first date this Friday night, but now thinks calendar can work in her favor.

"I look at the calendar and say it's a 50-50 shot I'd like him and if I don't it's not my fault," she said.

As for Dr. 13, while he loves to point out things like how Fidel Castro and Butch Cassidy were both born on Friday the 13th and notices when he checks into a hotel if his room number adds up to 13, he doesn't want people to misunderstand.

"I don't buy any of this," Fernsler said. "I'm just a math guy."