In the past, I would say a late interview would be anytime after March 15th, since that is when schools were required to offer acceptances to the amount of people for which they have spots in their class. All other acceptances would then be given when the earlier acceptees decline.
So this year, since the date has been moved to March 29th, I would say April 1st begins the "official" late interiews.
hehe, I have one on April 2nd and April 3rd, lucky me! <img border="0" alt="[Laughy]" title="" src="graemlins/laughy.gif" />
Then again, one could argue that anything after New Years is a late interview.