I got an idea after watching the My Little Pony: Friendship is Magic episode, Suited for Success.
The dresses are based on the ponies' dresses for the Galloping Gala, only in human versions.
The first and second one is Twilight Sparkle's Starlit Dress and Rarity's Princess Gown.
Twilight Sparkle and Rarity @Habsro/Lauren Faust
Dress Designs @Me