I think most programs and candidates interview from roughly the middle/end of October to the end of January.
I agree with previous posters that, other than doing one or two practice interviews before going to what you think may be your top choice(s), it really doesn't matter. If you are going to be memorable (either good or bad), interviewers and current residents will remember you regardless of when your interview is. Try to do what is most convenient for you.
Since I interviewed around the midwest and new england, I tried to do the programs that I would need to fly or make a long drive to earlier in fall while the weather was still half-way decent and local places later in the season. I even caught the fall colors when I flew into Vermont - which was beautiful.