For MIT, the interview deadline is December 10.
Do I just have to contact my interviewer by then, like, emailing him/her December 9th, or do we need to already have established an interview location/date/time by then?
It’s a contact date, but if you wait until the last minute, it might be hard to schedule a time that works for both of you.
Yes, we try to interview absolutely everyone who contacts us by the deadline, but every year there is at least one applicant in my region who writes to us on the 10th to say something like “I would really like to schedule my interview. My school breaks up for the holidays on Friday the 11th, and I am off for three weeks of skiiing holiday starting on Saturday the 12th. But I am back on the 30th and would really like to have the interview on New Year’s Eve.”
Keep in mind that ordinarily all parts of the application, including the interview report, need to be in before the deadline. In practice, the EC’s usually have a few days grace period, and a report received during the first week in January will still be fine, but the idea is to find a mutually convenient interview time. Please allow enough time to ensure that that can happen.