to dress up (to wear formal or elegant clothes)

to dress down (to wear casual clothes)

Eg 1:

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.

Eg 2:

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.

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Do you “dress up” when you go parties?

Do you usually “dress up” or “dress down”?