"This is what you do in Washington, isn't it?" inquired soprano Anna Moffo last night at a party to promote the gala opening of Wolf Trap's summer season which is advertised as "a star-studded evening of entertainment" on Friday, June 1.

Moffo wanted to be sure that what she was doing-moving around to meet most of the guests-was what she ought to be doing as the honored guest at an increasingly common phenomenon here, a pre-gala fete to stimulate interest and promote ticket sales.

Moffo had agreed, back in December, to chair the event. This meant not only lining up performers such as Shirley Verrett and Ben Vereen ("I went all the way through my Rolodex"), but also squeezing in a day of appearances here yesterday.

And at the party, which took place in the Georgetown home of the Wolf Trap ticket committee's vice chairman, Steve Martindale, she seemed to have her role firmly in hand. "Pardon me a minute," "but let me say hello to these people," she said, spying some new arrivals at the door. "I think it's part of my function."

Moffo had accepted her duties gracefully through a long day. She flew down from New York on an early shuttle, taped a promotion ad at radio station WGMS, appeared on Channel 5's "Panorama" interview show; had lunch at San Souci with Martindale and her husband, Robert Sarnoff (who made it down on a later shuttle), and then went to the party, which had to end early so she could catch a 9 p.m. jet for London.

But even so tightly planned a day produced surprises for which Moffo could not be prepared. One came when a member of the Wolf Trap staff informed her at the party that the setting of the Giannini-Shakespeare opera, "The Taming of the Shrew," in which she is to appear later this summer, would be switched from the Renaissance to the 1930s.

Grimacing her disapproval, Moffo grabbed the shoulder of a listener and declared, "I'm glad you're here just to hear this. I've had operas changed before. I did Violetta (in "La Traviata") once, updated to the 1890s. It meant I had to wear a bustle instead of a hoop, and every time I tried to sit down it bothered me. But I don't think I've had a change like this." The ensuing argument, put forth by the staff member, that updating would place the focus more clearly on women's liberation, left Moffo unconvinced.

Then later, during the obligatory speeches in the dark yard (including one by Interior Secretary Cecil Andrus), Moffo heard gala coordinator Helen Smith explain why publicity was needed: Ticket sales are lagging. For instance, 140 boxes have been sold, with 226 more to go; and there are similar problems through most of the house.

Following that, Moffo went upstairs to change clothes for her flight. She was off to Munich (after a two-day layover in London) to record the sound track for a film she will do with baritone Hermann Prey in August about the life of composer Franz Lehar. She had done her part for Wolf Trap for now. CAPTION: Picture, Anna Moffo, right, with Cecil Andrus and Mrs. Jouett Shouse; by Joe Heiberger-The Washington Post