When to see the northern lights?

The answer to this question is simple: as long as there is darkness, there is a chance to see the northern lights. If you are traveling above the arctic circle, remember that the midnight sun will be shining all day long.
As a general rule, the darkest months are the best (december-january), since the nights are longer. However, contacting the local tourist office is usually the best thing to do: they can guide you through the best moments to travel to the location, available tour operators or guides, gear rental and any other information that can be handy.

3 pictures stitched together. ISO 2000, f=2.8, 1 second exposure.
Taken in the Prestvannet lake in Tromsø, Northern Norway