from my understanding I don't think that we have a particular time we can purchase a Bitcoin because I know quite well now Bitcoin is something that is unpredictable and it may because of the price is low and you purchase Bitcoin maybe after sometimes the price will also increase so we don't have a specific time for someone to purchase Bitcoin but the thing is that you have to only wait when there is a bearish market you can purchase as you like for long time investment instead of short time investment
The best time to buy will be only in a bear market, but I am of the opinion that you will not be able to determine when the bottom will be and when to buy, so it is worth dividing your funds into equal parts, and start buying after a certain period, or on corrections, in my opinion. In my opinion, this is the strategy that can allow you to get the optimal average purchase price.
You can, of course, try to guess when the price will reach the bottom, but it will be like playing the lottery, just trying to guess. You may or may not be lucky, so having a clear plan will help you act without doubt, and get a good result.