Investing while the price is low sometimes lead to a good thing, but make sure you see a potential that can make the price grow, because it'll be useless if you buy low price coins because it's dumped already.
And of course, for me i hold my coins although the price is low, since it'll bring me loss if i choose to sell it. I prefer to be patience and wait until the price recover, at least i get my money back, even if i can't make profits from that. Don't need to be panic and sell everything when the price drops, because it's mostly normal in this crypto world.
And for the best time, i think you're the one who should determine it. Since you own it and you should know what you're going to do with it. Don't follow other people since you might regret it later if it doesn't meet your expectation.
To cope up falling prices is to stay calm, don't panic, instead think of creative ways. Fluctuation is part of the game since market is very volatile and unpredictable. Thus, it's very important to be mindful, and be aware that volatility is the real nature of bitcoin and all we need is acceptance. Do own research, be strategic, continue to learn, and gain as much knowledge as possible.