I think it is always the right time to invest in bitcoin because even if the bitcoin price falls from the day you invested in bitcoin if you HODL and wait for a few days the bitcoin price will recover and even make some extra profits on top of your initial investment. Investing in cryptocurrency requires wisdom and the ability to close eyes from the market for a while, if you don't have patience and wisdom to HODL you are likely to lose all your investment in a matter of days.
If the price is down from the price I bought, I will still hold and try to buy for some amount because no matter how long, the price will be back increased and sometimes, bitcoin gives me surprise by an increase for another high price so that means, my profit will be bigger than before. The key is to manage our patience by holding bitcoin and accumulating more satoshi to prepare for the next increases.
We already saw the bitcoin price movements a few days ago and in a few hours ago, bitcoin can increase to $50,900, which means those who bought bitcoin below $49k make a decent profit. This situation can be like this until the end of the year but that is just speculation to prepare to search for our opportunity to buy low and sell high.