You don't have to make a single choice as when to buy, the best strategy is to spend small amounts and spread the trades, for instance if you buy every week for 100$ while the market is crashing you can make sure that you are not putting all your money on one go...
This could actually be better than just putting it all in when the prices dip because you get more value out of your money as the prices continues to dip. I might try to employ this strategy in the future.
Maybe you mean about buying during a sharp decline in prices for a while and waiting for the price to return to better or exceed your desired prediction. It becomes something you believe in to see the good. Although everyone will think about the losses that are pictured in the head.