Yes, offers are made on the 15th. I was offered a spot into an MFA from the waitlist on the afternoon of April 15 a few years back. Other friends have been admitted to programs later than that date. Being waitlisted at a dream program while accepted to a maybe program is kind of a crap situation to be in-- but at the same time, schools are aware of this and the domino effect that can happen, and will hopefully try to keep you informed. If you haven't reached out to your waitlisted school, maybe do that to get an idea of where you are on the list, how many people haven't said yes yet, and to tell them that if they offer you a spot you will definitely accept. Hope it works out for you!