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 — the most times it can happen in one year.

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.