Duchess of Richmond's ball
The Duchess of Richmond's ball was a ball hosted by Charlotte Lennox, Duchess of Richmond, in Brussels on 15 June 1815, the night before the Battle of Quatre Bras. Charlotte's husband Charles Lennox, 4th Duke of Richmond, was in command of a reserve force in Brussels, which was protecting that city in case Napoleon Bonaparte invaded.
Source: Wikipedia — Duchess of Richmond's ball (CC BY-SA 4.0)