No one can predict the price, they just gamble. The best way to optimise your buys is to split your investment amount and try to buy a little every day/week instead of buying all at once (unless we reach 4000k or something like that hehehe FUD).
It is unknown if when were the market goes up or goes down so it only means that we couldn't tell when is the best time to buy because anytime is significant to the world of cryptocurrency. But I guess the market of bitcoin will go down beyond $6000 and it will be remain in $7000. We'll it's just my gut feeling.