As I understand matters, you can withdraw an acceptance before April 15th. After the 15th it's iffy, but frankly, if you don't want to be at a school, I'm sure they would release you from any commitment to attend -- a good program probably doesn't want people there who don't want to be there.
I'm not so sure about the timing thing -- it might be worthwhile to inquire at your schools whether students have until end of the business day or until the end of the 15th, full-stop (hence my half-serious joke about a nocturnal council). In some cases, I've heard of programs giving their admits extra days/extensions to decide, though at places with active wait-lists I think that practice is less likely to occur.