The best time to buy BTC in my opinion is right before the CME launch on 18th December.
There is obviously an example of CBOE futures launch on 10th December, when Bitcoin made one of the largest growth in % in 5 minutes time, I think the same will happen upon CME launch. I have a quick video proving my point, would be glad to hear your feedback:
https://www.youtube.com/watch?v=JsR9N-TDBWM