First day: suit and tie
Anytime you're going to be in the courtroom for proceedings/hearings: Suit and tie
Everyday: hang a suit and matching tie on the back of the door (you'll likely share an office with at least one other intern, so work it out as to where you'll store your clothes).
Every other day: whatever you feel comfortable in, trousers, shirt, tie and jacket (this is what I normally do as a clerk).
Just be comfortable in what you wear and don't...