It was as crisp as the crease in a schoolboy's new trousers, cool as the crunch of an icebox apple. It was autumn, the big chill, come to Washington a month early.

Even the telephone company's weather woman seemed revived. "Chilly," she chirped to anyone who called. "Unseasonably cool . . . 53 percent humidity," she said soothingly, as if she, too, had finally had a good night's sleep beside an open window.

Like groundhogs in the wrong season, much of Washington yesterday poked its nose out of air-conditioned burrows and found the weather unexpectedly, extraordinarily fine. Ten lovely degrees below normal, in fact.

"We woke up this morning and I could feel that cool air coming in the window," said Sharon Payne, as she stopped in Old Town for a snack. "My husband calls it the 'I can breathe again' weather."

The cool air, pushed into the area from Canada by Monday's thunderstorms, is expected to last through Friday, and maybe Saturday.

In the suburbs, temperatures last night were expected to plunge to the upper 40s, putting a crimp in the all-night serenades of summer's surviving crickets.

The signs of change were everywhere. The dawn patrol at Rock Creek Golf Course noticed yesterday that the gnats that dive-bomb the ears and noses of duffer and pro alike had suddenly disappeared. At outdoor cafes women in summery silks looked out of place, hugging their bare arms and edging their chairs into a weakened sun.

On the roof terrace of the Hotel Washington, china cups of hot tea steamed where tall icy concoctions had once stood. Great ceiling fans were still after a long summer of slicing torpid summer air.

And on the street below, 5-month-old Allison Hicks of Columbia, S.C., was wearing the first sweater of her life, a little yellow number her mother had tugged over her small arms in the morning.

"I'm glad we did," said Katherine Hicks as she and her husband headed toward the Mall. "She's not used to this weather." From deep within her stroller, Allison Hicks sneezed a tiny affirmation.

The normal temperature at this time of year is low to mid-80s, according to forecasters. "This is late September weather," said National Weather Service meteorologist Scott Prosise. "It's nothing unusual. We're just getting a sneak preview of fall."

At the Belle Haven Marina south of Alexandria, the brisk wind rocked the sailboats in their moorings, setting the halyards clanging like porch chimes. Wind socks inhaled and exhaled. The river was a September blue, and against the Maryland shore, windsurfers spat across the whitecaps.

The few with the good fortune to be playing after Labor Day made the most of it, but without much company.

John Michael, a 25-year-old University of Maryland student, was tying his board on top of his Volvo. "It's beautiful, you couldn't ask for any better." He looked around the deserted marina. "But nobody has the day off." He himself was headed for classes. "Is today Wednesday? Probably radio, TV, film. Either that or Soviet history."

For the first time in months, the chores of construction workers looked enviable. Gwen Acty, a 26-year-old laborer from Hyattsville, stood in the sun, loading dirt from the renovation site of the Willard Hotel. She wore a white T-shirt that read "If You Want The Job Done Right, Hire a Woman," and on top of that, a sweatshirt.

"This is just the way I like it," she said. "Not too cool, not too hot." Dirt flew, and Acty laughed, without so much as a bead of perspiration on her brow.

At a French restaurant in Alexandria, a blackboard outside the door announced the inevitable: "Last week for soft-shelled crabs." Inside, La Refuge chef Jean-Pierre Laurent sat near the kitchen door, flipping through food magazines and concocting the autumn menu.

"The weather gets colder, we begin to take out the cold soup and think about stew. This year, I will do game with honey."