This will fall under the 2016 timetable which isn't out yet. However, I wouldn't expect much to change on this route. You basically only have one option for the 26th of December and it looks like that:
https://gyazo.com/3064b8967664de3ca71380d87c2db2ec
It involves a relatively lengthy layover in Villach.
If you go on the 27th, that's your best bet:
https://gyazo.com/d60b5effb5206898c32cc92b8567b99c
Again, remember to check again when the 2016 timetable is out (usually at the end of October or beginning of November).
It's easily bookable via ÖBB.
Go here:
https://ticketing.oebb.at/inet/pub/en/ticket
Enter your travel itinerary and then at the bottom under "Who is travelling?" you'll see "Group Entry Form". That's where you enter your group's details.