Schools are required to accept at least 1 person per available seat by mid-March.
Many places choose to release all their admits by then, and will only accept off waitlist in April/May and beyond. Some however will admit only what they are required to for mid March and will continue interviewing and/or releasing decisions into late March and April.
April 30 is the official end of the normal cycle period when everyone holding admits has to commit to one school, after that they only changes in people's plans should be due to waitlist admits.