The Mushroom Ball in the Mushroom Kingdom is a special event that is meant to celebrate the upcoming coronation of Princess Peach. The Ball is mainly held in the Mushroom Palace (Peach's Castle) in the grand ballroom and several citizens of the mushroom world attend the ball.
The last time the ball was held, it was when Peach's father, King Toadstool was crowned king of the Mushroom Kingdom, and even a flashback was seen of he and his wife dancing together at the ball.
Princess Peach invites everyone to the ball, and sends Mario a special invitation to attend as her date. Everyone who attends the ball wears extremely formal clothing that differs from their normal attire.