I did this when I was between rotations during my third year. I went with my wife to visit her family and while we were there, I visited the program in her hometown. I had emailed the residency coordinator at that program, told her that I was going to be in town and that I wanted to visit the program for a day. She was very helpful. She set me up to go with some of the residents throughout the day. I spent all day there, from about 6am to 5pm, and got a pretty good feel for the program. It was nice because everyone was on 'normal behavior' vs. how it is on interview day when everyone is on their 'best behavior'. I got a good feel for the program without having to spend a whole month there.