No one will tell you the exact time when it should be done. I think that buying Bitcoin is best at a time when its price has dropped significantly. Selling Bitcoin is better when its price is high. And since its price is very variable it is easy to track.
That is the principle of investing, buy low and sell high. But its true, there is no exact time as to when people should buy bitcoin. Definitely it would be when bitcoin price goes down but how do we know. Bitcoin price is unpredictable. Just buy bitcoin when you feel like you have to buy it and sell it at your target amount.