The National Symphony Orchestra held its traditional season-opening party after last night's concert at the Kennedy Center. It was held in the Center's atrium three and half months after it was originally scheduled.

But if it had been given as planned on Sept. 26, a number of important people would have been missing -- including the entire orchestra then on strike and, conceivably, music director Mstislav Rostropovich, who joined his players on the picket line.

Orchestra president Austin H. Kiplinger referred to the situation in a brief speech, calling last night's gathering "a secord start to an orchestra which had its faltering steps in the beginning. But that was only a mere interruption." He called the belated party "a family occasion."

The program last night added a most unusual family dimension of another sort. Restropovich conducted the world premiere of a concerto for soprano, cello and orchestra by the French composer Marcel Landowski. Rostropovich both conducted and played the solo cello while his wife, soprano Galina Vishnevskaya, sang.

There was much enthusiasm for the composition, particularly from Vishnevskaya, who kept ecstatically repeating in English, while standing in the receiving line, "Fantastic! Fantastic!" to the composer.

Later, they all mounted the podium, and Rostropovich took command, declaring, "Always in our family Galina sings and I speak. Except sometimes after some vodka when I start to sing and she has to speak." At that point the microphone started humming and he broke off the occasion by declaring the hum to be clearly "an F."

Jean Cotte, of France-Soir, declared the premiere to be an example of the new romanticism that is being discussed so much in music. "It is a beginning of the movement for France." Asked how it would be received at the new avant-garde music cultural center, Beaubourg, under composer-conductor Pierre Boulez, he said, "So far as Paris is concerned Landowski regards Boulez as public enemy number one and it's probably reciprocal."