everytime, always, all the time, everyday, every week, you can go on and on. this means acquire bitcoin when you have the fund. because while you are thinking, the price could rice in the next minute.
Yeah you are right about volatile nature of bitcoin i.e. their prices continuously fluctuates in the market. So we cannot fix any standard time to buy bitcoin because of these variations. I will recommend to buy bitcoin when their prices are low in the market as compared to other cryptocurrency.