to dress up (to wear formal or elegant clothes)
to dress down (to wear casual clothes)
A: What should we wear at the wedding reception?
B: The reception is at a very expensive hotel, so I think we should dress up.
A: Should we dress up for the party?
B: I don’t think so. I heard it is a casual party, so most people will probably dress down.
Do you “dress up” when you go parties?
Do you usually “dress up” or “dress down”?