In Europe, a new timetable comes into effect each year on the Sunday after the second Saturday in December. This year, that is on 11 December. The booking horizon can be a lot shorter when booking trains after the timetable change. Trains of some railway companies are already available but others aren't. Most trains are opened for booking in the course of October and November.

I'd advise checking the SJ website every once in a while. That is also the best place to book reservations for this night train.

Unlucky for you the reservations/tickets for Swedish trains are only released until the 10th of December and unfortunately no one knows when the reservations/tickets for the 11th of December and onward will be released, but according to the information currently available it will not be before November. Once the reservations/tickets are released I guess that the demand will be high.

There is a lot of trouble with the planning of the time table for trains in Sweden due to late information from the companies performing construction work on the tracks. 

Ah this makes more sense! Thank you both for your quick replies! I suppose we will just keep checking! 

Some railways release early-and then have to inform all booked pax the times have changed or whatever. Some may allow you to give @mail and get on alist when this happens, but as anna from Sweden did not tell this, and she knows best,  I guess not.

To prevent this other railways will not release till its 123% sure that the times are right.

Also note there will not be that much to see around that time: polar nights, only round lunchtime it lights up a little that far north.