It's a good thing the party season starts in September, and it's a good thing September-like weather started last night. Because if it hadn't, the Kennedy Center might have been a sweatbox.

The National Symphony Orchestra concert started shortly after an electrical fire knocked out at least two transformers and, consequently, the air conditioning. And just before conductor Mstislav Rostropovich took the podium, there was some worry the fire would put out all the lights in the packed Concert Hall.

Guests were game for it. Maybe even a little excited. When NSO executive director Henry Fogel came on stage before the concert to announce there had been a fire and "there is a possibility if, when we turn on the stage lights . . ." the audience broke into laughter anticipating what he was about to say. But the stage lights came up and stayed up, and the audience applauded this confirmation of Kennedy Center electrical strength.

After the concert, Rostropovich had this to say: "Not even fire cancels us. Music more power than fire."

In typical fashion, he delivered that statement as he sprinted around the Kennedy Center atrium during the after-concert party bear-hugging the guests.

Rostropovich, of course, is a Russian e'migre' and is deeply troubled by the Soviet downing of the Korean airliner. "That's my personal tragedy," he said. "That's very deep. That's not coming out of me until I die. That's one of the greatest shocks of my life since I left Russia."

The conductor said he had gone to a Russian Orthodox Church here to pray for the victims. "I have not made any statement about the tragedy ," he said, "because that's over my words."

In honor of Rostropovich--Slava to the world--the reception had a Russian theme. The food included caviar (no, not the Caspian Sea variety, but some tasty domestic substitute), stuffed grape leaves, piroshki, lamb kebabs, blini for the caviar and sour cream.

"It's really Russian peasant food," said Barbara Cox, the NSO staffer who organized the reception and had the caterers follow her Russian cookbook.

Even the caviar?

"Maybe not the caviar, but the other food is. It's what the little ladies would bake."