When should I start studying for midterms?

For the past two years, I haven’t studied for any midterms. Freshmen year was fine because I got all A’s (and a B+ in Geometry Honors but we don’t talk about that). However sophomore year I got mostly B’s and B+'s on my midterms and I realize that I need to step up my game for junior year. My school is separated into four quarters and midterms start approximately at the end of December to early January. When should I start studying?

Early November would be the most preferable, as you can create a study schedule that easily hits every unit in every class without feeling like you’re cramming.

I would highly recommend the book by Cal Newport: “How To Become a Straight-A Student:
The Unconventional Strategies Real College Students Use to Score High While Studying Less”
It will help you with time management and how to be more efficient.