It was a cold day in August yesterday.

At what is usually the time of maximum steam-bath mugginess in Washington, the area was treated to what seemed like a brisk fall Sunday. Fairfax youngsters hoping for a final swim before school begins found their local pool too chilly to take the plunge. Frustrated vacationers on the Eastern Shore had to settle for a tan instead of a dunk.

At Dulles International Airport, the previous low for any day in August of 44 degrees was erased early yesterday by a 38-degree chill. At National Airport, temperatures sank to a comparatively mild 53 degrees, just surpassing the record low of 50 degrees set in 1874.

The cold snap was by no means confined to the Washington area: Record lows were reached in 31 cities around the nation. In Vermont, it snowed. It was 28 degrees in Thomas, W. Va.; 29 in Sault Saint-Marie, Mich.; and 23 in Old Forge, N.Y.

In western Maryland, police officer Terry Spear had to scrape the ice off his windshield before driving to his headquarters in Oakland, Garrett County, where the low was 28 degrees. Later, he said by phone, his wife called him to report that the potted flowers out on the porch had "turned a little black."

In the Washington area, there were few complaints about the reprieve from the usual August double-whammy of heat and humidity.

At the National Zoological Park, a spokesman reported that the animals were "out in good numbers" and seemed to be enjoying the cool. As for humans, the spokesman said, "they're just coming out here in droves, and a lot of them are running around in shorts."

On one hand, it was a good day for festivals. "It's glorious, delightful, just beautiful!" exclaimed Joyce Cappon, president of the Waterfront Washington Association, as hundreds of area residents enjoyed a weekend festival and fishing derby. "It's warm in the sun, it's just perfect," said Cappon.

On the other hand, though, it was not a good day for pools. At a private swimming pool in Annandale, manager George Staples reported that attendance, usually high this time of year, was noticeably down. "Everybody's sunbathing," he said. "It's just a little too chilly to go in the water."

Northwesterly winds blowing down from Canada and the Midwest, along with a succession of cloudless days allowing heat to disperse, were probably the two most important factors in the weekend cold snap, said weather specialist Larry Wenzel of the National Weather Service.

Dave Gustin, lead forecaster for the National Weather Service in the Washington area, said temperatures would probably begin to climb today. He discounted the possibility of any serious crop damage locally in areas other than far western Maryland, where subfreezing temperatures might conceivably have caused problems for farmers.

In the Midwest, however, where the cold wave hit hardest, a bumper crop of corn and soybeans was threatened. In Cadillac, Mich., the mercury sank to 26 degrees, and the cold threatened the tender late corn.