Best time to invest or buy into BTC is the down periods when doubts are in and right now though price is higher then the summer the price action is lagging.     You wont get the best price when gains are fast but because that gain is more immediate the majority seem to prefer this, ironically its more risky then just taking a long term view of the horizon for BTC and adding even in slow periods such as now.

Anyone in a muddle and free markets in price at least are almost completely designed to confuse the poor end user then just buying regularly is also more rational then attempting to guess every high and low per day per week or even per month.