The interview day will be a couple of hours of presentations, a talk from admissions/the dean, an OMM demo and tour of campus. Coffee and breakfast bars in the morning and lunch provided. During the tour you're free to ask your ambassador whatever questions you want. Then you'll do the interview and the day is done.
As far as weather goes, it's pretty dang cold and snowy right now. The buildings are very comfortable temp wise and you'll be taking the tunnels most likely so should only need to be outside to/from the parking lot if you drive. It'll be cold with just a suit, but admissions will have somewhere to keep your coat if you have like a peacoat or whatever professional people wear. Wear dress shoes. Thermal long john-type stuff would probably be very uncomfortable after the first couple of minutes inside, but that's just my opinion!