I think December is best - it's not too early, and not too late.
When I have interviewed before, I have been told that if you interview too early they can forget you, and if you interview too late, they may already have made up their minds.
1) You'll have more energy earlier in the interview season (so avoid January if you can, unless you have January off. I saved most of my pre-lim interviews for January--it was tough, but it sure beat being really drained for a PM&R interview)
2) It is also nice to have one or two interviews under your belt before you interview at your top programs.
3) It's really, really nice to schedule interviews in the same geographical region at the same time, as that can save you a lot of money. That was one of my big factors in when to schedule interviews.
So I really think it's just personal factors that come into play as far as when to schedule your interview--I really don't think it makes any difference to the program.