Should I ask for a confirmation of my interview if I answered late to the invitation? I have been short listed to be interviewed for admission to graduate school. A couple of days ago, I received an e-mail with four possible time slots, but I coudln't answer immediately because of a health problem. Indeed, a friend answered for me. My answer was a bit delayed but still on time, during the past weekend. The first slot was on monday morning but certainly, I choose the last slot to be sure I am going to be in the best possible shape for the interview. That slot is in 1 day and half and I haven't received additional information, neither a confirmation nor a plain rejection of my interview. 

Should I assume that the interview has been setled and wait next to the phone or write to confirm the interview? If I have to write, which is the best way to do it? I don't want to sound like harrasing the interviewer.
You should attempt to confirm the interview.  It's quite possible that somehow your email was somehow missed or there was a miscommunication.  The people doing this stuff are human and make mistakes.  Even if they are actually planning to do it, getting confirmation is a reasonable request.  If you don't hear anything, you should probably be ready by the phone at the appointed time, but don't be surprised if they don't call.