The best time to buy Bitcoin is “As soon as possible“.
In a few words, the deflationary nature of Bitcoin makes it always tend to incrase it’s value. Therefore, you can buy Bitcoin at any time, and from this moment, the tendency is that it gain more value than it had when you bought it.
Consequently, the sooner you buy Bitcoin, the longer it will be yours, and therefore the more value it will acquire.
Which brings us to the logical conclusion: Every day you wait for Bitcoin to go down in price to be able to buy, it’s a day when it will get even more expensive. If you expect it to get cheaper to buy, it will never buy because it will never get cheap.