there is no particular date in mind. Every DS sends their acceptances around Dec 1. They continue to interview more people until deposits counted Jan 15. Second round of acceptances is send Jan 15. Depending how full they are they will send more interview invites. At this point some schools will be forming wait list and some schools will be 1/3 full so they will continue interviewing into spring just each time acceptances announced you have less time to decide- from 45 days first time to 2 weeks at the end. Some schools like Marquette do not like to interview more people so they will choose second round from people already interviewed. Generally many schools are full by the end of Feb. They might interview more people in March for waitlist....many large expensive schools will interview for actual spots way into March and April. There is no exact number as while schools have pretty good idea what their yield is usually is on each date, it is still different.
Case has overacceptance applicants couple years ago - by second round there were like 30 people over? which required lots of wheeling- dealing to do and they still ended up with 15 people over the class size.