I would really suggest that you try and do some shuffling so you can go on the 15th. March is pretty late, and I wouldnt want to give up a decently on-time interview for one that late in the season. Honestly, unless it was the birth of my first child, I would be heading to that interview next week. I don't know what your reason for not being able to go is, but if it's something silly like a test in a class or work reasons, cut and go! An acceptance to such a great school would be worth the sacrifice to me!
You'll want to decide that quickly - you can't reschedule interviews online if it's less than one week from your interview date. I had to reschedule my first interview with them, and since it was less than a week from the date I had to speak to the admissions office and request they make an exception for me. They graciously did (which I was so thankful for), but that might get harder to do as the season gets later.
You may see odd dates open up sooner than March in their scheduling system if people cancel/reschedule, so keep checking it religiously throughout the day. But I would tend to echo scowdeva's comments - do everything in your power to go to the date you have next week! They make decisions pretty quickly, and you'd likely know by early February!