In this market, there is no "best" time to actually buy or invest in something, anytime may be a good and best time, especially if you have a lot of capital to spare, another one to consider is the coin you're investing, and in this case, with Ethereum; I say that it's good coin to invest at anytime, similar to Bitcoin. It's because they have good values and support groups that continuously help it grow, if not, retain what value they have, and most importantly, surge and get back the value that it lost.