In order to choose when to take the MCAT, you must first ask yourself a series of questions such as:
When will I apply for admission to medical school?
How long do I need to study for the MCAT?
What materials will I use to study for the MCAT?
Where will I register to take the MCAT?
Will I take the MCAT just once or will I want to retake it?
Each question is explained in detail here https://www.mcat-prep.com/mcat-blog/how ... -the-mcat/