Hi there! The number one rule when planning on writing your MCAT is feeling 100% ready. Do not take the test if you are not feeling confident in your knowledge. You must be scoring in the 90th percentile during your practice tests before you take the actual exam.
For the majority of traditional applicants, the summer after your second year presents a great option to take the MCAT. This is for two main reasons: First, by this time you will have covered a majority of the content through prior coursework. Second, taking the MCAT in the summer lets you dedicate more time and energy to preparing when you are not encumbered with trying to keep up with your other coursework and optimizing your GPA, which should always remain a priority.
Most students who do not want to take a break between undergrad and medical school take the test after their sophomore year or in their junior year of college. If this is your plan, keep in mind that you will have to juggle your college coursework and exams with MCAT preparations.
If you are taking a gap year before med school or a longer break between college and medical school, you still want to take the MCAT by April or May of your application year. Hope this helps and best of luck to you!