There will be no specific time to buy and to sell. It will depend on the situation. But the point : buying at lower price, selling at higher price. This is the basic role that we can gain profits through trading any crypto coin (including Bitcoin trading).
Sure, we need to do analysis to get the appropriate time for entry (buying). Mostly people will say that we buy at the correction or dump. Then, selling at the next pump. Unfortunately, we may be trapped if we choose the wrong time for entry. That's why it is need to do a deep analysis before considering to buy Bitcoin or altcoins.

So the best time is to buy when I have the money~
Of course.

You don't need to think about trading if you have no money at all.
It is the main requirement, you actually shouldn't forget it.