What should I expect when checking in to a new duty station one day after Christmas?

There will be a reception center where you will check in, they will do a bit of paperwork and get you a barracks room to sleep in while you finish in processing and arrive at your actual unit.

This link has specific information

http://www.campbell.army.mil/Arrivals/Pages/Kalsu.aspx

If you drive there, ask the gate guard for specific directions. If you fly in, there will be a representative at the airport (or USO) that can arrange for you to get a bus ride onto post and more than likely the bus will take you straight to the in processing center. Just make sure you arrive before midnight (0001).

If you have anymore questions, feel free to ask!

Basically exactly what MSG (Join to see) said, to the letter.

Your orders should tell you exactly where to go. And because you're at a major installation, everyone will be able to tell you where to go if you can't find it on your orders. Basically there should be a reception centre. You go there, report in, and they'll direct you what to do next. Congratulations and good luck!