It's a matter of optimization, but I would hope 6 or 9 months of full freight insurance isn't make or break for your FIRE plans. If so, then you might be playing it a little too close.
So, for example: quit at the end of the year, to transition to ACA, but then skip out on a bonus paid out in February? Or, pass on a "voluntary separation" window that happens midyear? Probably not good moves.
To answer your question: signing up for ACA is effective the following month, if you sign up before the 15th of the month, or 2 months after, if you sign up after the 15th. So, signing up for ACA at the normal open enrollment time for employer insuance plans works OK.
However, loss of a job triggers an ACA "special window." Even a planned, future loss of a job (I.e. job is gone by time ACA plan would start) So, as far as the ACA is concerned, they don't care when. You would, of course, have to take into account the income you have earned. I was coming onto ACA from overseas, and landed with an ACA plan in place. Of course, you could always COBRA for a month or two, if you need time to transition, or even if you mess up the timing,
Be careful about that income, too: it is not your spending. If you are spending from cash, your income is $0. If you are spending from taxable investments, your income is your capital gains. So, be careful not to fall into Medicaid territory accidentally, or even worse the gap area for states that have not expanded Medicaid.