The Soviets and the Americans made it official last night: General Secretary Mikhail Gorbachev will bring his wife Raisa to Washington when he comes next month for his third summit meeting with President Reagan.

"They just informed us she was coming," said State Department spokesman Charles E. Redman at the Soviets' celebration marking the 70th anniversary of the "Great October" socialist revolution. "I don't know that there was ever any uncertainty, but you just never assume things until you know."

And even when you know, apparently, you never assume things.

At the start of the evening at the Soviet Embassy, Liana Dubinin, wife of the Soviet ambassador, was noncommittal about whether Mrs. Gorbachev would be making the trip.

"I hope," she said.

Some 2,000 guests, unknown pounds of caviar and countless bottles of Georgian wine later, what reluctance the Soviets may have felt earlier about discussing Mrs. Gorbachev's plans had disappeared, and Liana Dubinin was cautiously talking about some of the sights the Soviet first lady might like to see here.

"It's difficult because we have only three days, but she likes very much art," she said to a suggestion that the National Gallery might be worth considering.

Liana Dubinin's eyes danced at the idea that the Soviet leader might like to visit the National Air and Space Museum. "Now, it depends on the State Department, but I like this museum," she said. "I was with my granddaughter there. It was wonderful."

Jack Courtemanche, Nancy Reagan's chief of staff, said Mrs. Reagan is talking about the visit already -- "She's ready right now." A Soviet advance team will arrive in the middle of next week, he said.

Secretary of State George Shultz's arrival confirmed what everybody had already guessed: that this year's party would be more warmly received than those in recent years.

"Ever since Afghanistan our representation at these functions has not been high. Now it's an indication of a relationship that's going better, plus it's his relationship with {Soviet Foreign Minister Eduard} Shevardnadze," Redman said of Shultz's presence.

Bulgarian Ambassador Stoyan I. Zhulev agreed: "The new face of relations between the East and West holds many more expectations and tonight's audience here represents the hopes that the process continues."

Soviet Ambassador Yuri Dubinin squired Shultz through the crowd like a proud father, then whisked him into a small room where they could hoist their glasses to each other in semiprivate.

"The secretary said he was stressing the importance of spending quite a bit of time with the Congress," Sen. Alan Cranston (D-Calif.) said later. "Dubinin is well aware of the necessity of doing that."

Cranston, as majority whip, said "we can't assume" ratification of an arms treaty will be easy.

"But subject to seeing the fine print -- and I assume the fine print will be fine -- I will work as hard as I can with others to get it through. We can't assume there won't be strong opposition -- 10 to 20 senators are pretty apt to take it on directly," he said.

Said Sen. Alan K. Simpson (R-Wyo.) of ratification: "I would certainly hope we can get that. I think it will be the first step in a series of things that will be very productive on into next summer, on to START {Strategic Arms Reduction Talks}."

Czech envoy Miroslav Houstecky called the agreement Reagan and Gorbachev expect to sign "a breakthrough ... but I don't think it should rest just with INF {intermediate nuclear forces}. If this course continues, both East and West can prosper."

As for glasnost, in Czechoslovakia, "there was always glasnost," he said. "People had plenty of opportunity to express anything they wanted to within the framework of the national front. Different organizations -- trade, youth, women's -- could vent all their demands and grievances." Houstecky stopped short of describing Czechoslovakia as a model for Gorbachev's reforms. "You cannot expect that anybody will claim his own country's model is applicable to the other countries," he said.

Czechoslovakia has been among the least open of the Soviet bloc nations since the "Prague Spring" of 1968, when Soviet troops crushed a Czech attempt at liberalization. Earlier this week a senior Soviet ideologist suggested the Kremlin may be ready to reassess its suppression of Czechoslovakia's 1968 experiment in openness.

The huge picture of Lenin that hangs over the staircase provided the backdrop for what seemed an unending receiving line engulfing the Dubinins. Coming through it were the predictable diplomats and uniformed military attaches and at least one who was unpredictable, Prince Bandar,

ambassador of Saudi Arabia, which has no diplomatic relations with the Soviet Union.