If your worried about your interview date then just call back each week and see if they have any cancellations. Usually you can move up a few weeks because people have accepted at other places and cancelled.
As far as the interview process, it should be very relaxed. I am not interviewing candidates the weeks mentioned above, but I did interview two candidates today. The interviews last about 20 minutes and most of those interviewing will make it very easy on you. I do know maybe one or two students that take their interviews a little too seriously, but the students have very little input on your admissions. The faculty interviewers that I know are wonderful. When you get to the school in the morning they will give you a paper with your assigned interviewers. You will have time during the campus tour and lunch to ask students about those you interview with. I hope your interviews go well.