Good, Fast, Cheap: Pick Two Out of Three Project Management Triangle
It is the difference between optimal and acceptable.
Optimal for a rational, logical, non-task competing application cycle is having MCAT taken by May so score is available prior to first transmission. This is
staring on time and thus being able to complete the many necessary tasks so they dont compete with each other for time, priority, and effort. Or the equivalent of keeping up with work in multiple courses from the beginning of the term. Taking the above "Good, Fast, Cheap" idea, you can have it Good and Cheap (in terms of effort) because it doesnt need to be as Fast as you have the most time here to spend.
Acceptable is getting everything done that you need to by the time you need to. That will mean having competing priorities such as prepping for the MCAT and trying to polish your primary and trying to prewrite secondaries and checking on LORs. This is the equivalent of not keeping up with your work during the term and now having to complete papers or prep for multiple exams that are due the same time. Its like spending all-nighters to get things done. So in the "Good, Fast. Cheap" idea, you must have Fast and it must be Good, so now it isnt Cheap in terms of effort/energy. In fact, you may not have enough energy to get it Good while trying to get things in on time or you sacrifice quality to get it Fast.
I call the second "Acceptable" concept the "Summer Cascade" where all the tasks start cascading down to the deadline. It is not an optimal method.
So back to your specific confusion. For best results, have MCAT done by May. In terms of timing, have everything in by mid September. And remember this is mostly rolling admissions. To get everything done with a later than May MCAT and have it all (Primary, Secondary, MCAT and LOR) in by September you risk not being good, And you dont get in by being fast, you get in by being good. I said all this before in this thread (see my self-quote below).
One more thing, taking a rushed, late MCAT risks current and all future applications. Like herpes, a bad MCAT stays forever. And everyone you apply to will now you carry this.