Everyone knows Washington slows into a sweaty, swampy, steamy ghost-town in August -- but U.S. News and World Report has decided to do something about it.

The news weekly, which is based in Washington, is moving to a four-day work week for August.

All U.S. News operations will be on a "compressed schedule" of 40.5 hours over four days, "to give our employes more time off with no loss of pay or benefits," Fred Drasner, the company's president and chief executive, told the staff in a memo distributed this week.

"Washington is slow in August," Carol O'Leary, director of personnel, said yesterday, noting that Congress and the White House shut down for much of the month. "The news is not as active as it is the rest of the year."

The experiment will affect about 500 employes -- about 400 of them in the Washington area, she said. About a dozen employes on the copy desk and in manufacturing have been working four- or three-day weeks.

All operations, from news to manufacturing to sales, will try to use skeleton crews on Mondays and move as many people as possible to a Tuesday through Friday work week. The magazine goes to press between 9 p.m. and 11 p.m. Fridays.

Instead of a five-day 37.5 hour week, plus daily one-hour lunch breaks, the magazine will try a four-day week, with 45-minute lunch breaks. Department heads will be given the flexibility to schedule employes as necessary to get the job done, the memo said.

U.S. News is not the only business in town to notice the doldrums of August.

"When Congress is away, there's nobody here to play," said Mel Krupin, owner of the Connecticut Avenue restaurant that bears his name and caters to the downtown business establishment. Krupin said his business falls off by about 25 percent in August.

The Greater Washington Board of Trade, which acts as a sort of regional chamber of commerce, doesn't even hold meetings of its 91-member board of directors in August because most members are on vacation, a spokeswoman said.

Of course some companies see increased business in August: Air conditioning drives up summer electrical consumption by about 13 percent, compared to winter use, according to Potomac Electric Power Co.

The decision to try four-day weeks at U.S. News is "an attempt to give people a little bit of a break," said Kathy Bushkin, director of editorial administration. "But the key priority is to put out a quality magazine on time."

The news department will continue covering the news seven days a week, but will "try to go along" with the new program to the extent possible, Bushkin said. Congress will be recessed from Aug. 16 to Sept. 7. President Reagan is expected to spend those three weeks at his California ranch, while the White House air-conditioning system is being rebuilt.

Many New York advertising agencies take Friday afternoons off, so the U.S. News advertising department might take Fridays off rather than Mondays, O'Leary said.

O'Leary and Bushkin said the four-day schedule will not save money. Computers, air conditioning and other basic operations all will keep running, so overhead costs will be basically unchanged.

Nor will it increase costs. It is seen by management as a way "to provide a benefit to employes that doesn't necessarily cost money. Maybe it will yield good will," O'Leary said.