When is the best time to travel to northern Europe?

It is good to travel to northern Europe in winter, because winter is the best time to see the aurora. At this time, there will be many activities to play in northern Europe, such as dog sledding, snowmobiling, glacier hiking, living in an igloo, chasing the aurora and so on. However, the best time to go to northern Europe should be from May to September every year, because the temperature in northern Europe is just right during this time, which is relatively suitable for domestic people. When you go to northern Europe, you must keep warm.