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?
I have a feeling you're trying to be philosophical here.
Ideally you should buy stuff when they are cheap and you expect the price to go up later. On the other hand you got to start somewhere and there's really no easy way for normal players like us to know how long spikes and plunges would last. So basically, just get in there. Buy so you have a starting pile and then add more as you see fit. At least then, your initial purchase would have already earned at least some interest.