I went back in September, it's REALLY informal. Get there early if you want pizza, otherwise it's chips, cookies, and coke (all bad for your teeth...). I arrived late because my flight got in late and it was raining hard. There are about 10 students who put it on, and they chat with you about the school and answer any of your questions. Jeans or Khakis are both fine- no one will care what you are dressed like (there are no professors or admissions people), just students. I came in shorts and a golf shirt, and I didn't feel underdressed.
Have fun! Good luck with interviews!