Most people take the MCAT at the end of their Junior year, by then they have (nearly) finished physics, organic, and many biology courses which cover material which is on the MCAT. I took my in the summer right after I finished my junior year. If you take it later you are definitely running a risk of if you getting a low score you may not be able to take it again and get your application complete until very late in the game. By taking it early, you can still retake if you need to. I believe the aplication process opens the beginning of June, like June 1st. The earlier you can get all the information to them, the better. Ask your letter writers as soon as possible. The bulk of the application process occurs the summer after junior year, before senior year, with interviews during beginning/mid senior year. You should meet with your pre-med advisor at your university, he/she should be able to tell you in greater detail.