Denmark, Norway and Sweden in Dec.

It will be very dark early in the day.

And late in the day too!

Short daylight hours and pretty cold for outdoor activities - unless that's your thing. Cities will be fine to visit, no different from cities like New York, Toronto, Chicago, etc in the winter.

Don’t drive. The roads can be treacherous and some will be closed.

What are you looking for? Can be a great time to see the Northern lights. Plenty of winter activities available too, although a bit early in the season for many of them.

Denmark is not as bad for weather as Norway and Sweden.
I have been to Copenhagen in January and the weather was cold, but about the same as southern Germany, where we lived at the time.
I would not recommend going to Norway or Sweden in the Winter.

Yes it will be dark. You have a better chance of seeing the Northern Lights if you go above the Arctic Circle - Tromso or Alta in Norway. Best time to see the Northern Lights is in the dark of the moon (New moon phase). We went late January, early February and had about 3 hours of twilight in Alta - but very beautiful.

RE: I would not recommend going to Norway or Sweden in the Winter.

Unless you are a skier then the best time would be late February into March. Longer daylight hours than December and the ski tracks would be marked and set. There is always thing to do depending on your interest.

Edgar,
Good point, if you like skiing, then Norway would be great for skiing in the Winter. Of course, your days would be very short.

Enjoy the skiing and outdoor sports
Many of the slopes are well-lit.
May want to try curling?

Denmark is not as bad for weather as Norway and Sweden.

No, it is a lot worse. The damp, chilly winter weather in Denmark and southern coastal Sweden/Norway is a lot worse than the proper winter weather you get more north/inland in Sweden and Norway

I would not recommend going to Norway or Sweden in the Winter.

I would. If you enjoy skiing there are some great ski resorts in the Scandinavian mountains, including several used for world cup and world champoinship races (both cross country and downhill). They also tend to be less crowded than the ski resorts in the Alps. And for spring skiing, Riksgränsen in northern Sweden usually keep the slopes open until late May.

If you don't like skiing, you can go dogsledding in the wilderness or take a snowmobile tour (available everywhere). Or have dinner on a mountaintop while watching the Northern lights. And if you're brave enough, go swimming :) Or maybe a stop in Harads, a small village in northern Sweden that is home to two quite unique hotels, Arctic Bath and the Tree Hotel. And of course, there is the more famous original Ice Hotel.