Next year, when conversations turn to the "Summer Nights by the River" festivities at the Kennedy Center, you are likely to hear, "Nobody goes there anymore; it's too crowded." The idea of dancing, sipping and snacking on the terrace after a concert has already caught on to the point where it is distinctly crowded for promenaders who want to stroll to such tunes as "42nd St.," "Puttin' on the Ritz" and "The Blue Danube," though the Mostly Mozart crowd (a civilized lot) do leave a good, clear place in the center for serious dancers.

In fact, the week-long Mostly Mozart Festival, which offered spectacular performances last night with Raymond Leppard conducting and Janos Starker and Ken Noda as soloists, is one of the most civilized things that happen in this town. The music is ample, beginning with a preconcert recital at 7:30 -- a royal feast last night with both soloists playing Beethoven's "Bei Mannern" Variations and Starker gloriously alone in Gaspar Cassado''s brilliant Suite for violoncello solo.

The program was not actually mostly anybody, though Franz Joseph Haydn scored highest with two works performed. But any program that contains the Concerto in D minor, K. 466, is qualitatively mostly Mozart, particularly when it is performed as well as it was last night. The orchestra tightened up neatly for this work after some minor ensemble problems in Boccherini's Symphony in D minor ("La Casa del Diavolo"), and Noda played with a maturity, a restrained use of his powerful technique, and a sense of the music's subtleties, that belied his young age and even younger appearance.

Haydn's Cello Concerto in D is a pleasant, modestly distinguished work, but in Starker's hands last night it suddenly became great music. He took a minute or two to warm up fully in the first movement (and indulged in a few little portamenti en route), but by the time the cadenza arrived he was playing at a white-hot level of intensity and demonstrating what everyone in the audience must have known: that he can perform technical and expressive marvels without even bothering to look interested in the music. The slow movement was a model of singing without words, the finale a demonstration of brilliance beyond what one might have expected in the music. The applause was tumultuous and prolonged but not one second or one decibel more than he had earned.