I would say REALISTICALLY the end of January, though that's HIGHLY not recommended. I only that since end of January is also when many Congressional nominations are announced.
Technically, the deadline for CFA should be the deadline for the application, which is around the end of February.
Last year, I did mine around December, but it put me behind schedule when I received my nomination (I took the CFA after I received the nomination). Because of that I had no opportunity to receive the primary slot, as I was not 3Q'd.
Do it as soon as possible! Good luck!