Buying Bitcoin can be done at anytime that you want but you have to be smart about how you're investing because investing with the perfect timing gives you an advantages over others and investing when the market is down is better than investing when the market is at its peak that most whales that invested when the market was at its lowest will be selling to take profit which can also cause some corrections. For someone that has no interest in taking profits, you mightn't need to time your entering into the market perfectly but for someone with that intention in mind, timing the market is necessary to avoid missing out on the profits. Buying at the price you have in mind will be good for your mental health as then you'll know that you bought at the price that you always wanted to buy and didn't fomo into the market or buy out of pressure.