Anytime you want, just go buy some bitcoin as the price constantly get higher and higher even there are some decline.
depend on your purpose, do you want some sort of profits in short period? and then you need make precautions before buy it, read news and new update of bitcoin in every section so you know if the price would decrease instead of increase.
My purpose, of course, is to earn profit from it. That's what everyone's goal is, I think. Right now it's the price is going up and I think it's not yet the time to buy. I'm thinking about it as a trading type, from fiat to btc. I guess you buy from a lower value so you get the best value from your real money.