It was a little like the first day of school: The dresses were crisp and fashionable, the talk lusty and familiar.

Indeed, summer must be over.

Political celebrations. Charity balls. Embassy glitter. From deviled eggs at the White House to the thicket of city politics, last night Washington was in business again.

But the jovial ambiance of reunion was dampened early in the evening with the news that a legendary princess and an embattled president-elect had died within hours of each other in different parts of the world.

"Oh no! I'm stunned. I can't believe it," said singer Pat Boone during a White House reception when he first heard Princess Grace of Monaco had died. "All I can think of is that there's a cloud over the sun today."

Across town at the Ambassadors Ball, Secretary of State George P. Shultz waved away reporters who were asking about the killing yesterday of Lebanese President-elect Bashir Gemayel. He did, however, whisper to Secretary of Defense Caspar Weinberger for a minute or two in the middle of the room.

"It's too early for an official reaction," said Weinberger. "It's a very shocking and sad thing. I have personal sorrow. I met him over there. He was a great friend of the U.S."

"The president was shocked . . . We hope it doesn't upset the pursuit of orderly peace in the country," said presidential counselor Edwin Meese.

Nonetheless, after three months of the traditional sticky social slump, "reentry" was off to a blooming start in every crevice of the city.

Marion Barry raised his glass to a huge throng at the Capital Hilton in honor of his anticipated mayoral victory, while Montand crooned sweetly to an audience jammed with Washington's glamor class at the Kennedy Center.

At the Washington Hilton, Shultz made his tuxedo debut with patent leather shoes at the Ambassadors Ball, a Hollywood-like premiere of Cabinet members and double-parked government cars that snarled traffic for hours.

And on Pennsylvania Avenue, Pat Boone and Andy Williams partied with the Reagans at a White House reception to launch what was perhaps the most perplexing affair of the evening -- The Washington Charity Dinner. The dinner -- held afterward at the Sheraton Washington -- was a $500-a-head fund-raiser with no connection to establishment Washington.

Which made the White House reception that no one would take credit for arranging all the more mysterious.

"Joe Canzeri," said a well-placed White House source when asked how a little-known Montana organization finessed the reception.

"Not true," said Canzeri, a former aide to Reagan now involved in public relations. "It was probably Pat Boone and Efrem Zimbalist Jr. . . . They campaigned for the president."

Around town, hundreds filed back from the Vineyard, the Hamptons, French chateaux and Rehoboth to no fewer than 10 parties scattered throughout the city.

"It's been a fine respite," said Attorney General William French Smith. "But I guess it's that time again."

In the familiar spirit of Washington, the most sought-after social events were those stacked with Reagan administration officials. Last night, it was the Ambassadors Ball.

And the fall social pageantry has only just begun.

"There are 12 benefits just this month," lamented one PR executive. "Organizers are going nuts trying to sell tickets."