There will never be a perfect time to invest, more than likely, unless you're extremely lucky, you will buy and experience both dips and peaks. If you can afford to lose your money invest it, if you cant, don't.
At the moment prices will be lower than they have been due to people cashing in to pay for their Xmas shopping. I would expect that, and this is purely speculation, once the banks reopen, after the Xmas break, prices will probably bounce back up. But, that said, they will probably drop down again for new years/around new years due to people over spending when celebrating the date changing on the calender.
This advice and prediction were just so spot on ha!
There is no magic formula. If money flows into a coin it goes up, if it flows out it comes down. Follow crypto news and general customs the world practices (such as holidays, weekends) and you might be able to make better trades.
Buy when stuff is going down sell when it is going back up..
If you're looking for a perfect time to invest is you're going to buy coins on holiday season like what happened recently, last December was the hardest time for investors as most major coins are dropping together with Bitcoin but that's not actually the worst part because it's the perfect time the traders were waiting for, cheap coins are best investments when you know and expect that coin will pump soon.