The 15th is the deadline when people will accept or reject offers. If your at the top of a waitlist, that'll get you a spot. For the programs aimed at having a well rounded cohort, they'll have to look at who accepted and those they need to keep the cohort well rounded and contact them with offers, if they reject, then someone else may come up, this can only happen after the 15th.
http://www.cgsnet.org/
Schools that belong to the CGS have an April 15th deadline, unless otherwise indicated by your school. I think people should have made up their minds by now.