Diana Ross at the Kennedy Center last year. (Jacquelyn Martin/AP)

Talk about a wedding singer! The celebration for Washington Performing Arts Society chair Reginald Van Lee and longtime partner Corey McCathern lasted three days including a welcome party Friday at Sax restaurant, Sunday brunch at the Kennedy Center. . . and a surprise appearance by Diana Ross.

More than 700 people attended the couple’s wedding Saturday night at the National Building Museum; the ceremony took place on a bridge built over the center fountain. After dinner, toasts, cake cutting and a dance performance, the grooms announced one final surprise: “Miss Diana Ross!” For a moment, guests weren’t quite sure if an impersonator was coming on stage. Then the diva herself — wearing a red sequined gown — swanned on, and the crowd surged against the stage for the 15-minute concert. “She was fabulous,” one onlooker told us. Unclear exactly how the couple scored Ross for the wedding (she’s said to be a friend of Lee) or how much it cost to hire a Motown legend to entertain at a private wedding. Lots, we’re guessing.