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You will either be accepted on the 15th, or sent to a smaller group who will decide if they want to consider you in the future or reject you. As these groups meet according to the group members' schedules, you could get a notice anytime in the month. (This was what I was told last year, I'm not on the ad com, so I can't verify it 100%, but it's at least a close approximation of the truth). Until after April 15th (decision day for applicants, when you have to give up multiple acceptances and choose one school), the consider later catagory is called hold. After 4/15, they see how many spaces they have left, and go through their hold list. They accept some holds, waitlist some (the waitlist is broken into two groups of roughly 50 applicants each, if you are in the top group, you have a good shot of acceptance), and reject some. So if you get a letter on the tenth of January, it's most likely to be a hold. Few people who they interview are clearly a bad fit for the school, so most interviewees initially get placed in the hold catagory. There is a theoretical chance of a hold being accepted before April 15, but it is highly unlikely.