Grand Galloping Gala
The Grand Galloping Gala is an annual ball thrown in the Equestrian capital of Canterlot. It is an invitation only event attended by the pony aristocracy in regal attire. Princess Celestia personally greets every pony in attendance at the castle's great hall. There is a ball room featuring a live orchestra and a private garden that the ponies drift through as they mingle; and every year the Wonderbolts put on a performance.
Every year Rarity makes new clothing for the attendees. When the girls all received invitations to the Gala courtesy of Celestia herself, Rarity makes all six of them dresses that they will wear in episode 26, The Best Night Ever.