"Dress robes are worn by wizards on formal occasions."
From the Story
Discovered in Book 4, Chapter 10, Dress Robes
"During Harry's fourth-year, the Hogwarts school list states that students will need dress robes. Dress robes vary in style; the robes Mrs Weasley buys for Harry are more or less the same as his school ones, except that they are bottle green instead of black. Ron's dress robes had to be bought second-hand, so there weren't many options for Mrs Weasley to choose from. His robes look like a long, maroon, velvet dress, with a mouldy-looking lace frill at the collar and matching lace cuffs."