As long as you're not buying at a current ATH then I generally think most any time is a good time to invest in BTC.
This month all digital currency prices dropped dramatically, this is because the number of people who buy bitcoin and other currencies so that the exchange rate decreased. The key if you play treding you are patient, because it does not close the possibility of bitcoin will rise many times over the previous price. When bitcoin prices are falling as dramatically as they are today, bitcoin players will sell them for less.