Some day in the future, you will realize that the best time to invest was not today, but some other day in the future, when prices were low. But with the information that you have today, the best time to invest is still today, because you do not know when prices will be low, and you do know that the longer you hold your investment, the more time it has to grow.
Toughts?
The best time to invest is when market price is down, however, since altcoin or even bitcoin price is volatile, there's no way to tell or even predict when will bitcoin price go up or down. You just need to do your research and be informed on what is going around bitcoin because current events regarding crypto also affects the market value.