Prospective students usually arrange to take the MCAT a year before their intended matriculation into medical school. For example, if you are eyeing medical school in 2023, then you can take the MCAT in 2022 or before.
Since there are no academic prerequisites for the MCAT, you can take the MCAT as early as your freshman, sophomore or junior year of undergraduate studies as long as you feel that you are ready. You may consult with your pre-health advisor. (Read: How to determine when to take the MCAT)
One important consideration is whether or not you will re-take the MCAT. Should you feel that you might take the MCAT more than once, it's ideal to take the MCAT early in the testing year. This will give you time to receive your scores and make a decision.
Another consideration is readiness. Have you thoroughly prepared for the content and skills that are required for the MCAT? Ideally, you should start your MCAT prep at least six months prior to your test date. (2022 MCAT Test Dates)
The pie chart below shows that 2/3rds took the exam in August and September. This suggests that having most or all of the summer available for MCAT preparation may be one factor in gaining a high MCAT score.
Next: How many times can I take the MCAT?