It was one of those cool days, so unexpected in August, that could almost make one forget all the other wretched days of summer in Washington, the enervating steaminess, the hot soaking rains, the sleepless nights that bring no breeze. All over the city people awakened in fine moods, if for no other reason than that it was cool.

Among them was Frances Goldsteen, 55 years in the fur business ("It's a nice, clean business. No one undresses."), who said to herself: "Gee, that's nice. It's cool. Maybe we'll be busy." On such days, as Goldsteen knows from long experience, the daydreams of a certain kind of woman turn to mink and fox.

By 4 p.m., four such women -- considered a lot in the fur trade -- had appeared in the green-carpeted crystal-chandeliered salon on Garfinckel's third floor, where Goldsteen works. And in the departments where dresses are sold, the sundresses that seemed so promising in May and June had been banished to the sales racks, hidden behind the wool sweaters and corduroy skirts.

Outside, the Good Humor man at 14th Street and Pennsylvania Avenue NW wrote the day off as a loser. Hardly anyone was buying ice cream. "It's close to the worst," said Andrew Traldi, 16, working as a Good Humor man to help pay his expenses at Columbia University, where he will be a freshman this fall.

"The only thing that could possibly be worse is if it were raining." In deference to the weather, Traldi had cut his own consumption of ice cream from four Chip Candy bars to two. He amused himself playing with Rubik's Cube and doing a fill-in-the-blanks celebrity quiz that gave such clues as: "If the star of 10 married a longtime singer/actor/vaudeville player, she would become a handful of flowers." The answer: Bo Kaye.

A blond woman in a white flowered dress and big sunglasses sat on a bench, painting the Washington Monument in watercolors while her husband watched. Louise Hammond, 67, from San Francisco, said she felt inspired by the breeze and the Rodin exhibit at the National Gallery, which she had just visited.

"It feels San Franscisco-ish today," she said. Just as she was putting a lovely green wash on the trees around the Monument, a man in a red woolen cap walked past. This, like the breeze, inspired the impressionable Mrs. Hammond, who remarked, "Oh, I think I'll give someone a nice red hat."

The Shriners, here from all over the country for a convention, were everywhere, their tall velvet rhinestone-studded hats glittering in the sunshine. Zelodis Vaughn, a broad-shouldered 48-year-old deputy sheriff from St. Louis, Mo., looked particularly regal under his hat as he relaxed on a bench with a friend, Lucy White, a medical transcriptionist from St. Louis. Vaughn seemed to take the weather in stride, but White couldn't get over it; she kept commenting on how nice it was.

A guard at the Hirshorn Museum, Millard Schafe, stood against the railing of the sculpture garden, looking at Henri Matisse's four magnificent bronze backs, dreaming of the West. "San Francisco, that's the weather today. I love it out there -- anywhere north of Big Sur. The redwood forests, Monterey, Carmel . . . "

Charlie Chilton, forecaster at the National Weather Service, welcomed the change in the weather. "The summer can be sort of monotonous," he said. "This is a right pleasant day." The low temperature was 62 degrees, the high was 76 . The lowest temperature recorded here for Aug. 17 was 51 degrees in 1902, the highest, 97 degrees in 1968. Yesterday's cool air came from Central Canada, and Chilton said it probably would remain through Wednesday. He said it reminded him of fall, 36 days away.