Generally speaking, http://bahn.de/ is very good to find train connections, even if you are not traveling through Germany and/or cannot book that particular route through the site. Usually, you could therefore check in advance if the train runs on the days you are interested in but it won't help in this case.
The problem is not Thello's 120-day booking window (Christmas is already less than 120 days from now) but the bi-yearly timetable changes in European train networks. Timetables for the winter season (all the way to June) will be made officially available mid-October.
What you can still do in practice:
- Look in the past, the timetable for December 2012 is still available on bahn.de and could give you an idea of what to expect.
- Contact Thello directly, they probably already have an idea of their plans/slots for the coming season (if you do, please do come back and let us know!).
See also the news for September 2013 on seat61.com