AS SOON AS POSSIBLE!
I am currently undergoing the application process and let me tell you that I wish I had applied as soon as applications were being accepted. I know it seems like it shouldn’t matter that much and that many people like to think everyone gets an equal chance at acceptance, despite when you submit an application. The fact of the matter is that the sooner you apply, the more likely you will get admitted to medical school. Even if you have a excellent application and ‘know’ you’re going to get in somewhere, trust me when I say that the waiting game is excruciating. Seeing others receive acceptances when you have yet to attend your interview (or even receive an interview invitation) is very effective for increasing your blood pressure!
So remember, apply EARLY!