Last year, when it was not a "special year," there were several schools that interviewed into late March and even up until May, and offered acceptances to those interviewees soon after (I have a friend who interviewed at Stanford in April and was accepted about 3 weeks later). If you get a post-March 15th interview date and you are interested in the school, you might as well go -- you might end up getting an outright acceptance, or you can end up in a very favorable position on the waitlist (and most schools do have significant waitlist movement).