Wednesday night a yellow-and-white striped tent covered the courtyard at the Iranian embassy - symbolizing, as its arrival always does - the beginning of the summer social season for Ambassador Ardeshir Zahedi.

And to kick it off, Zahedi was host to an intimate - no more than 150 - cocktail party for singer Beverly Sills who last night along with the New York City Opera Company performed "The Merry Widow" in a benefit for the company.

"Where's the caviar?" asked Sills' operatic colleague, Alan Titus, as he arrived to survey the two buffet tables flanking either end of the courtyard pool.Well, there was none, although guests hardly went hungry.

At each table, costumed chefs group up fresh meat for the steak tartar at one end while they dished up sauteed shrimp in garlic butter at the other. In between, guests chose from a variety of delicacies, including assorted cheeses, candied tomatoes and fresh strawberries lavishly strewn mid-table and banked by melons.

Inside, plates of petits fours sat under an enormous bouquet of cymbidium orchids, roses and carnations. Pineapples covered with strawberries served as centerpieces on the scattered tables.

Zahedi was hardly still a moment as he dashed about filling plates for guests or mixing the steak tartar he later fed to Sills and her husband, Peter Greenough.

Most of course were anxious to chat with Sills about her recent announcement of her plans to retire in 1980. "Oh, yes," she laughed, "the reaction to it did surprise me. But gracefully, most people reacted to it by saying 'Why so soon?' instead of 'Thank God.'"

At one point with an accordion and violin serenading them. White House inflation adviser Robert Strauss huddled at length with Washington Star publisher Joe Allbritton. Strauss maintained the conversation concerned Texas politics, although Allbritton pleaded weightier matters. "Strauss is going to the Kentucky Derby, so I asked him to put $10 on Affirmed for me. Actually, I would have bet only $2 but since he's going that far, I hated to be a cheapskate."

Meanwhile, among the guests were Bunny and Phillip Buchen, Sen. Birch Bayh (D-Ind.), Val Cook, Dorcas Hardin, sculptor Lolo Sarnoff and the new French ambassador, Francois de Laboulaye. It was his first time at a Zahedi party. Taking the opportunity to wander through the glittery premises, De Laboulaye pronounced the whole affair "beautiful, I am," he said, "full of admiration for how my host entertains."