On April 12, 1858, Mrs. Senator Gwin, wife of Senator William Gwin of California, gave a fancy ball which was the talk of Washington Society for decades afterwards. This ball is mentioned in many memoirs of the era. Most recently I have read a wonderful account in Virginia Clay-Clopton's memoir, A Belle of the Fifties (available for free download from Google Books in pdf and epub formats). The New York Times for April 13, 1858 also gives a comprehensive account of the ball.
Of note in the New York Times article is the following account of Mrs. Greenhow's costume and character at the ball: "Mrs. Greenhow, of Washington, as a Housekeeper of the Old School, was an admirable representative of a gossiping, agreeable person, acquainted with everyone present, and with everything that was going on." Presumably this is the same Mrs. Greenhow who was later a Confederate spy.