If you want to know what is the best time to buy bitcoin you could actually try on joining on the group where they could announce possible changes and analysis on the price and the changes in the stock and volume of bitcoin on the market that might help you on deciding whether to buy, hold and wait for possible opportunity that could come in trading.
There's no need to panic or do it fast, it requires patience, thorough research, and be up to date. It's recommended to check the price status in bitcoin wisdom or coindesk and asses yourself if you take the chance to buy or wait for another dump. For me, the best time to know when to buy is when the price is low, and I prefer to hold it to avoid big losses. Pump and dump has always been its trend, and I believe that recovery will happen, and that's when I can sell it and gain higher profit. Moreover, it still needs strategy and knowledge to decide how low is low before we can decide to buy and how high is high before we can sell it.