How would I be able to get from Yangshuo to Longji Rice Terrace on the 24th of December and back?

There are only two direct buses from Yangshuo to Longji. The earliest departing time is at 08:30 and the latest is at 13:30. And the bus will run in both directions at the same time. The ride takes about 4 to 4.5 hours and CNY 70 per person in a single trip.
If you want to go there and back in a day, in my opinion, it is impossible because you may get the Rice Terrace at 12:30 even if you can get the earliest bus and it is hard to get on the last back-trip bus. So you are advised to spend a night there.