Every schools is different. Most will send out enough acceptances to theoretically fill their class. However, there will be many withdrawals, so approximately 50% of the class gets filled. There are also a handful of schools that will almost fill their class up by December. You will hear of some schools that hand out enough pre-december interviews that they don't have to interview more people after december. They can just choose from an extensive waitlist or NTBC list. Again, every school is different, but there will be a whole bunch of post december interviews given out, as well as acceptances. The advantages to pre-december interviews are the ability to hear back sooner and you can be reviewed for acceptance multiple times. But post-december interviews are perfectly fine and can just as easily result in acceptance. Everything I have said comes from reading 2009 school acceptance threads. They are very informative and I suggest checking them out.