At many universities, those trying to avoid a gap year will most commonly take the MCAT during the spring/early summer of their junior year, but it all depends on the timing of when you will finish your MCAT coursework. For example, I didn't finish physics until spring of my junior year, and took the MCAT in early June of that year, and successfully applied that cycle and was able to go straight through without needing to take a gap year.
Stephen, EK writer and instructor, Emory M1