The National Symphony Orchestra's seventh annual Summer Mozart Festival began last night with a wedding ceremony--an elaborate affair on the Millennium Stage with lavish Colonial-era costumes, dancing and pieces of wedding cake for members of the audience. Staged by the Virginia-based Living History Foundation, which has come across the Potomac for every NSO Mozart festival since the series began, the ceremony re-created the kind of wedding celebrated in America in the 18th century.

The ceremony fit this year's musical theme, "Mozart and Marriage." Weddings were a significant source of income for Mozart and other 18th-century composers, responding to demands for religious music to play at the ceremony and party music for the reception. Most of the music on last night's program was relatively unfamiliar, including the rather conventional Te Deum, K. 141, composed when Mozart was 14, and festive excerpts from the opera "Ascanio in Alba," composed when he was 15 as part of the festivities for a royal wedding.

The centerpiece of the program was the "Haffner" Serenade, K. 250, composed for the wedding of a friend of the Mozart family, relatively familiar music but treated in an unfamiliar way. Christopher Hogwood, conductor of the program and director of the festival, "deconstructed" the serenade, as he said in his introductory remarks.

The hour-long work was not played straight through from beginning to end, as is common today; segments of it were detached and distributed throughout the evening--as well might have been done back when the music was new and used as a background, not listened to with solemn concentration by people sitting in silence. As usual in Mozart's serenades, a concerto was embedded in this one. The three movements of the violin concerto were played with an elegant solo and elaborate cadenzas by the NSO's associate concertmaster, Elisabeth Adkins.

Other segments of the program called on the skill of two groups specializing in 18th-century performing arts: the Washington Bach Consort Chorus and the exquisitely costumed New York Baroque Dance Company.

Both groups were featured in the seven excerpts from "Ascanio," which were taken out of the complete opera and arranged in an Italian-style serenade, a form that included both singing and dancing. The chorus, representing "Spirits and Graces" in some numbers, "Shepherds and Shepherdesses" in others, sang praises of Venus, the Goddess of Love, and of the "lofty hero" who was being married. The New York troupe, four couples, danced in elaborate patterns that hinted at a narrative subtext; sometimes representing Graces and sometimes the bride, the groom and their admirers.

Elsewhere in the program, three minuets were extracted from the "Haffner" Serenade and played as a dance suite, with stately and graceful dancing by the New York Company and another violin solo for Adkins. To conclude the program, the NSO (reduced to the size appropriate for an orchestra playing at a lavish 18th-century wedding party) had the stage to itself, performing the conclusion of the "Haffner" Serenade with the precision and polish Hogwood usually elicits from it.

The festival continues tonight and tomorrow with chamber music, vocal and instrumental this evening, and excerpts from two marriage-related operas ("The Marriage of Figaro" and "Cosi Fan Tutte") tomorrow night, as well as the great Mass in C Minor, which Mozart wrote for his wife, Constanze. Other attractions will include lessons in 18th-century dance and several lectures on the theme of marriage in Mozart's time and in Mozart's life.

CAPTION: New York Baroque Dance Company members minuet to Mozart.

CAPTION: Veteran conductor Christopher Hogwood at work "deconstructing" Mozart at the Kennedy Center.