“It’s a good holiday for business,” Mr. League says. The Greensboro, N.C., gumshoe has already scheduled five infidelity investigations for Tuesday, and plans to add two part-time sleuths to his staff of four to handle the demand.

Valentine’s Day is the biggest single 24-hour period for florists, a huge event for greeting-card companies and a boon for candy makers. But it’s also a major crisis day for anyone who is having an affair. After all, Valentine’s Day is the one holiday when everyone is expected to do something romantic for their spouse or lover — and if someone has both, it’s a serious problem.

“If anything is going on, it will be happening on that day,” says Irene Smith, who says business at her Discreet Investigations detective agency in Golden, Colo., as much as doubles — to as many as 12 cases some years — on Valentine’s Day.

Martin Investigative Services in Anaheim, Calif., has been booked up for Valentine’s Day assignments since the end of January. Founder Thomas Martin, a former agent of the U.S. Justice Department, says the firm, which charges $95 an hour, will handle 14 to 20 suspected infidelity cases that day, nearly double its usual load.

One of Mr. Martin’s Valentine’s Day clients is a doctor whose wife, also a doctor, aroused his suspicions when she told him she would be changing her regular daytime shift on Tuesday and instead working until 8 p.m. Like virtually all private detectives, Mr. Martin won’t reveal his client’s names. He says private eyes from the agency also will be following an attorney who told his wife he can’t have lunch with her on Valentine’s Day because he’ll be in court; the wife, also an attorney, knows that her husband always gets a lunch break in court.

The Institute for Divorce Financial Analysts, a Southfield, Mich., trade group of professionals trained to review divorce settlements, says filings typically spike in mid-February. “It’s so consistent I can’t deny a pattern,” says Natalie Nelson, a divorce financial analyst in Boulder, Colo.

Indeed, divorce lawyers say they frequently turn up evidence of Valentine’s Day duplicity when they review financial documents. Credit-card receipts from restaurants or purchases at fancy jewelry stores are the most common giveaways, says Heidi Harris, a partner at New York law firm Sheresky Aronson & Mayefsky. New York attorney Raoul Felder concurs: The kinds of purchases documented for Feb. 14 “give an indication of how serious the relationship is,” he says.

Christine Gallagher, a 43-year-old writer in Los Angeles, was so incensed after she caught her boyfriend cheating on Valentine’s Day that she launched a Web site called RevengeLady.com, where she gives advice on how to get back at people. Ms. Gallagher was dating the man, whom she declines to name, for over a year when he told her he had to go away over Valentine’s Day to visit a friend dying of cancer in Switzerland. Ms. Gallagher spent the holiday alone at home with her 180-pound mastiff, Thomas.

‘Classic Conflict Day’

It wasn’t until several weeks later that Ms. Gallagher learned the truth. As she was out walking Thomas she was approached by a woman who said she had just returned from a vacation in Italy — with Ms. Gallagher’s boyfriend. Before coming up with the idea for her Web site, Ms. Gallagher broke up with the man, then found an unusual way to get back at him: She unscrewed the driver’s-side door panel of his beloved Audi coupe and stuck a marble inside, figuring that the rattle would drive him crazy. Sure enough, it did. He took the car to mechanic after mechanic until one finally found the marble — and a little note Ms. Gallagher had included: “So you finally found it, sucker.” Ms. Gallagher says her ex-boyfriend now lives in New Zealand; he couldn’t be located for comment.

Indeed, planning a “business trip” that falls over Valentine’s Day is a typical mistake cheaters make, says New York detective Stephen Davis. Mr. Davis handled a case in which a husband took his mistress to Florida for a two-day holiday, claiming he needed to be away for work. Though it was part of an ongoing case, the Feb. 14 rendezvous was “one more nail in the coffin,” Mr. Davis says. The detective verified the tryst with photos. “It’s a classic conflict day,” he says.

This year, Valentine’s Day spending is expected to reach $13.7 billion, an increase from $13.19 billion in 2005, according to the National Retail Federation. But clearly, some of the dough shelled out that day won’t go for proclaiming love, but for verifying it. Just in time for Feb. 14, a New York company called Tru-test Inc. has released an $80 DNA collection kit that it says will help detect whether a partner has been intimate with someone else by analyzing suspicious stains. “I thought this was a good time to do it, because this is when people want to see if a spouse is cheating,” says kit creator David Vitalli. Manhattan detective agency Beau Dietl & Associates, which charges clients $100 per detective per hour for surveillance, says it expects Tuesday to be such a busy day that the firm will need to call in six part-timers to work on adultery projects.

The firm’s surveillance expert, Sean Lanigan, says he worked on a Valentine’s Day case last year for a married client who was planning to be out of town with his wife — and wanted to spy on his mistress. Mr. Lanigan says he followed the girlfriend to a card store where, using a special pinhole camera built into the button of his shirt, he videotaped her buying two Valentine’s Day cards. Her next stop was a store where she bought a dress using the client’s credit card — and finally at a hotel to meet another man.

Ruth Houston, author of a book called “Is He Cheating on You? — 829 Telltale Signs,” says she generally recommends against spending money on private detectives to catch cheaters because the indications are so easy to read. (Sign No. 3 under “Gifts”: He tries to convince you he bought expensive chocolates for himself.) But Valentine’s Day, she says, is an exception. “All the cheaters have to make contact that day,” says Ms. Houston. “This is the one time you’re not going to come up empty.”

Lauren Gorence, a 24-year-old medical assistant in Boulder, Colo., says she didn’t pick up any signs her boyfriend was sneaking around on her… until Valentine’s Day. When her boyfriend didn’t show up for a planned dinner that night, Ms. Gorence called his best friend, who said the boyfriend was out to dinner with a woman but he didn’t know where. Ms. Gorence waited outside her boyfriend’s house until 1 a.m., confronted him and never spoke to him again. (She declines to name him.)

Ever since then, Ms. Gorence says, she doesn’t attach any significance to Valentine’s Day. For the past several years she’s spent Feb. 14 hanging out with friends and plans to do the same this Tuesday. “It’s an overrated holiday,” she says. “I think every day should be just as loving as the next.”