The decision of when to buy rests on the person who is making the purchase as it would depend on a matter of factors; Someone who is buying in for the long term hodl does not bother about short term price changes and believes that in 10+ years the value is going is to much higher.
Someone who is not buying for the long term are usually more picky about when to buy and would not want to purchase when the price is on the way down. This is much more risky and would require some technical analysis and some luck as well.