I'm going to be in Stockholm over the festive season and will need to use public transport on 26th December to travel from the city centre to the airport.
What level of service can I expect on 26th December?
I have tried checking on their website for you and can't find anything out of the ordinary on there.
So I am going to go by the timetables and how it has been previous years. The 26th of december is a public holiday in Sweden and as such there will be the same amount of service as on a normal sunday. Which for subways usually means every 10 minutes in the middle of the day and less in the mornings and evenings. But there is still a good level of service as everything is running just with fewer departures.
You can also use the website to search between places you would like to go on the 26th already since the timetables are already updates.
Update: I just called the customer service to ask and the 26th will use the normal timetable for sundays.