"dressed tonight as though they were attending a performance at the Metropolitan Opera House"

The Metropolitan Opera House is is considered to be one of the most prestigious opera stages in the world, as well as one of the largest. It's on Broadway in New York City. Those attending a performance at the Met often wear floor-length gowns and tuxedos to openings nights or important events. Regularly, women will wear skirts and dresses with heels while men wear suits or blazers and dress shirts.

 

Sources:

Metropolitan Opera House website

Wikipedia: Metrolitan Opera