The best time to invest is when there's a huge amount of panic. The more that everyone thinks there's no chance of Bitcoin ever recovering and that alts will take its place, the better a choice it is to buy then.
Most people are too afraid to buy at the time the price gets beaten to the ground real hard. People rather buy at the time the price is going up, because a market that's going up gives them the confidence to actually start buying.
Or in some cases people tend to wait too long before buying as they are waiting for the price to fall down even lower. Risk here is that you miss out on a great buying opportunity in case the market doesn't go down any further.
People should just pull the trigger and refrain from waiting. You don't have to go all in ~ just use 10% of your capital each time, and if the price happens to sink deeper, you get to benefit as you can just buy another portion at lower cost.