I wouldn't take it during the semester if you can avoid it. The best time is at the end of a summer where you have time to spend studying (even if you're working). Taking it during classes is just not a good idea because it's a zero sum game - time spent studying for the MCAT must come from time spent studying for classes. The second best time if you can't take it in the summer is in January, when most people are done with finals and have four weeks or so to study intensively. I took it in April and I would not recommend it.