Everyday could be a good time to buy bitcoins as long as you have enough money for buying it and you are decided to buy bitcoins. But still there are some consideration that many are still waiting for the right time to buy when the price of bitcoin is some kinda falling down. And in this situation if will do that then your wait will just make you more lose a lot of money for the price is now increasing.
I agree assuming that bitcoin price have linear growth. But we all know that it does not happen. Bitcoin have several dips along the growth. So it would be best time to buy during those dips. there are some indication whether bitcoin price will go up or down. Just focus on the news and wait for some bad press and prepare your fund to buy more bitcoins.