I bought a bunch BTC when it was $17000, then bought a bunch when it is $14000...then the price dropped until 2 days ago. I bought a bunch again when it was 6000ish...haha, now the price went up...so, really, when is it a good time to buy? When it dropped for a while and it rose a little bit, then drop again, then buy?
For me, the best time to buy bitcoin if the sales become lower and rise again. In that time you need to buy bitcoin to become your price bigger. If you want to earn more, you should hold it until the sales become bigger in that way you can earn more money