I've read some quotes about the Gröna Lund being open in December (at least there is supposed to be a Christmas market there) but the official website is strangely silent.
Does anyone know if the Gröna Lund amusement park is open in December?
According to the German Wikipedia, the park is closed from mid-September to April and the Christmas market was discontinued in 2009. The calendar on the park's website supports this - it shows the park as open every day in August, only Thrusday to Sunday in September, and only concert events (which are held on the premises while the rest of the park is closed) afterwards.
Gröna Lund is closed during Christmas. As far as I know they have never been opened during this time of year. I think your Christmas market reference may refer to one of:
Both Skansen and Stortorget are nice but if you want to combine with amusement park activities you'll have to go to Gothenburg.