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 by the spring you want to apply. For example, if you want to apply after your junior year of college, it is recommended to take the MCAT by the spring of your junior year.
For most applicants, taking a mid-to-late summer MCAT will hurt their applications because most medical students accept students on a rolling basis.