The train times for most railway companies change every year on the second Sunday in December (10th December this year).
They are not finalised, and therefore not posted, until 1-2 months in advance. Also, the timetables are not all finalised and released at the same time, some routes are released before others. But, when the times are fixed, it will be for the complete 6 month period.
AFAIK there is no way to be automatically informed when they are released, you will just have to keep trying.
You can assume the times will be 90%+ unchanged. There is no "special" timetable for the Christmas period. Some railway companies (the Swiss and Germans) fix their times and publish them for the whole 12 months, December to December.
This problem affects Christmas and New Year travel every year for people who want to buy tickets a long time in advance.