Program with a start date prior to August 1st (for classes, research rotation, etc.) may require you to make a decision by April 15. If you accept their admissions offer, you are expected to withdraw from any other programs that have accepted you by that date. However, you may still keep your application alive at programs that have waitlisted you. For programs that have a start date of August 1 or later, the May 15th deadline applies.
Regardless of the deadline, program directors may pressure you to make a decision early (i.e. before the appropriate deadline). This is contrary to the spirit of the AAMC guidelines for managing admissions, but I have never heard of action being taken against a program that engaged in this behavior.