I don't know for sure, and it depends on the school, but it seems to me that the greatest amount of movement occurs between late May and mid-July. After that it slows down, but there still is some. I think the reason is that since people can't hold multiple acceptances after May 15th, there is a lot of shuffling around initially since people who were holding multiple acceptances withdraw, causing people to be pulled off waitlists (who then withdraw from somewhere else), which causes a domino effect. It takes awhile for all the spots to get filled, then things settle down a bit. But this is just my impression; I don't really know. I have also heard stories of people getting pulled off waitlists three days before classes start, but I think that's more the exception than the rule.