Email and ask admissions what the typical attire is for interviews - in my experience they have pushed for business casual because they know all-day events like this are tiring and they want you to be comfortable. Don't bring a boyfriend to planned events. For the interviews, have a list of several questions ready just in case there are gaps in your conversation. I would have at least 2 specific questions about the particular research they have done - it shows you cared enough to look into what they are doing.