As soon as the applause died down last night, most of the audience in the Kennedy Center Concert Hall poured out to the riverside terrace, where a moonlit fountain was waiting, the weather was ideal, and the Potomac River (always better-looking at night) sparkled darkly between Washington and Theodore Roosevelt Island.

Before the Concert Hall had emptied, people were already dancing on the smooth marble pavement to the sounds of the Carmelo Pino Trio, and the opening night of the Mostly Mozart Festival had modulated smoothly into its final movement.

In its sixth year at the Kennedy Center (and its 20th of existence at Lincoln Center), Mostly Mozart has already become a beloved tradition with tickets harder to get each season. Hardy music lovers can parlay their tickets into five hours of varied musical entertainment, beginning with a preconcert recital, continuing through a formal, two-part concert and ending with dancing on the terrace until well after midnight -- as long as the dancers are willing.

The evening that ended so idyllically began with a bit of a musical shock. David Bar-Illan, who was also the piano soloist in Mozart's Piano Concerto No. 18 in B-flat, opened the preconcert recital with an interpretation of Beethoven's "Moonlight" Sonata that was unconventional but highly convincing in its first movement. The moonlight in this interpretation was that of the trio in Act I of "Don Giovanni," where the Commendatore is dying -- a scene not of soft-edged fantasy but of horror and tragedy. In the first violin part of that music (beginning at measure 176 in the Dover orchestral score), Bar-Illan has found triplet figures similar to those that have hypnotized generations of music lovers in the Beethoven sonata, and his performance evokes the atmosphere of Mozart's scene. With a change in the balance of inner voices, a tightening of the tempo, Beethoven's dreamy music becomes sinister; his wistful melody assumes the atmosphere of a funeral march. Right or wrong, the interpretation is fascinating and disturbing -- and it carried intense conviction last night.

In the Mozart concerto, Bar-Illan played with immaculate technique and perhaps a shade more emotional intensity than is comfortable in this well-mannered music.

Two other soloists, flutist Carol Wincenc and harpist Heidi Lehwalder, glittered in the Concerto for Flute and Harp, K. 299, as well as in recital selections by Donizetti, Gossec and (possibly) Bach. They are superb musicians and play together with precise coordination. The harp sound was sometimes lost in the orchestral texture -- but Mozart probably intended that, at least when he reinforces the harp sound with pizzicato strings.

The program opened with Mozart's ballet music, "Les Petits Riens," and ended with his "Paris" Symphony. During the first work (which supplies any listener's normal annual requirement of gavottes), one could hear the orchestra, under the expert direction of Gerard Schwarz, pulling itself together after a long spell of wintry nonexistence. In the final symphony, it sounded fully like the orchestra familiar from past Mostly Mozart festivals -- one of the finest chamber orchestras in America