clothes

Day 1: jeans and a shirt given to you by panhel
Day 2: jeans and a nicer shirt and shoes, still casual but put-together
Day 3: skirt and top or dress (or dress pants if you're not into skirts and dresses)
Day 4: nice dress/skirt/dress pants, should be nicer than day 3, lots of people wear heels but that is by no means mandatory
Bid Day: jeans and a shirt, but your new chapter (!!) will give you a shirt to wear once you accept your bid
Basically the days increase in how fancy they are. Personally I don't think there is any reason to stress out a lot over outfits. It's definitely important to look put together and like you made an effort, but most chapters, including mine, are really there for your personality (possibly with the exception of one or two chapters (@aphi lol)).