There's really no need to worry, although with it being your first interview, you're going to be nervous (sorry, it's just how it is, ha). I interviewed back in September and am probably going to attend (there was actually a luncheon for admitted students today, but it was cancelled due to snow and ice). The only drag to the day is waiting in the "fishbowl" for about three hours while everyone's interviews wrap up. But I'm sure you'll love PCOM, and good luck with the interview!