When I was interviewing this winter, most places interviewed dec/jan. Many would tell you to come on a given date, but you could call and change it to another date. A lot of the "more competative" programs like to interview on Saturdays/Fridays and there is a lot of overlap between their interview dates. Hopkins, Duke, MGH, Brigham, UCSF all had intervews scheduled for the last 3 weekends in Jan. (altough hopkins had earlier interviews too and i'm not sure if UCSF had earlier ones).
So what I saw many people doing was getting a calendar from Nov. to Feb and calling the programs they had a good chance at and those they got interviews at and asking for their interview dates that way they could plan better.