There is no specific time to invest in bitcoin or in any alt coins because it was a long term investemnt.
Since the price of bitcoin is going up and down i think this is the good time to invest in bitcoin and hold until the price is going up again.
On this time if we buy in low price there is a big opportunity to earn huge profit if the price of bitcoin  recover on its high level price. Always monitor the market board to see how the price going on.