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.
You can also try to divide your money in 5 stacks, and buy in different days 1/5 portions of the amount of Ethereum and Litecoin you want.