(all) dressed up
Dressed formally, perhaps more formally than usual. You need to be dressed up for this event tonight—a suit and tie is appropriate. Because I was all dressed up for the birthday party, I was embarrassed when I saw that the other guests were in shorts and T-shirts.
*(all) dressed up
dressed in one's best clothes; dressed formally. (*Typically: be ~; get ~; get someone ~.) I really hate to get all dressed up just to go somewhere to eat.