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Advance tickets are not usually released for sale until the exact timetable is published, because they are on specific trains and things get very messy if you sell tickets on the stopping 15.15 train and then only run a fast 15.45 one instead.
In principle, the new timetable comes out on December 12th, as you have noted. But the Christmas period is usually heavily disrupted by engineering work, and the timetables for those days, which are known in advance to be disrupted, do not get finalized until later.
From their website, there is one set of major work currently posted in this period, primarily affecting Edinburgh - Dundee - Aberdeen. If you are expecting to travel on those routes, I would definitely anticipate some disruption.
But it is possible that more work will be announced. I don't think it's possible to say for sure when that will be or when those days tickets will be released - in an extreme example, Avanti (possibly the worst of them) haven't yet released tickets for ten days from now(!) I think you have a decent chance of knowing later this month, but I wouldn't want to promise it...
Upvote:4
In Britain, Advance tickets are generally not sold until the timetable has been updated to incorporate any changes due to engineering work (this is different to other countries' railways, where tickets are released a set period in advance and then you are notified of any changes).
Whilst it doesn't look like there are any engineering works that will affect your journey, clearly ScotRail have not yet released Advances for the date you want to travel, and so you will need to wait.
You can sign up here to receive an alert when they do - however, due to the way that The Trainline's alert system works, you should search for a more popular journey such as Aberdeen to Inverness, as you will only receive an alert once Advance tickets are found in a search that someone makes - and very few people are likely to search for a journey from Aberdeen to Thurso.
Also, note that you may be able to save money by buying a combination of tickets rather than one through fare - e.g. Aberdeen to Inverness and Inverness to Thurso. There are ticket selling websites that will calculate this automatically for you, e.g. TrainSplit (though note that TrainSplit charges you a share of the saving).
This is known as "splitting" and is perfectly legal, in fact it's quite common. Your rights are essentially the same as if you had one through fare, with the exception that your train must stop at (or you must change trains at) the station(s) where you change from one ticket to the next. Again, sites like TrainSplit will take care of this automatically for you.