Applicants typically take the MCAT after they have finished their prerequisite classes for medical school because these classes usually cover most of the material on the actual MCAT. This means that students are typically taking the MCAT after their sophomore or junior year of college. It is important to remember that schools usually do not accept scores that are older than two or three years (depending on the school). A good general rule is to take the MCAT one year before you apply. For example, if you want to apply after your junior year of college, it is recommended to take the MCAT the summer after your sophomore year.
You can still take the MCAT in January, March, April, or June of the year that you apply, but that will require you to study while you are taking normal courses. This can hurt your academic GPA as well as MCAT performance if you do not balance your time properly. If you want to take the test in July, August, or September of your application year, keep in mind that your application will not be early. For most applicants, taking a mid-to-late summer MCAT will hurt their applications because most medical students accept students on a rolling basis.