Besides activity from the sun, two factors will maximize your chances to see auroras: clear skies and darkness. In Northern Europe nights are dark mostly between September and March. During the last week of August and half of April it can be possible to see auroras, but the time each night is very short. So if you are travelling to mainly see the lights, I’d say September – March.
Near both equinoxes (September/October and Feb/March) the skies are usually the clearest. Between November and January is the darkest time, offering very long nights.
The Moon phases will affect your aurora experience as well. A Full Moon is naturally bright and therefore overpowers some details and weaker colors in the auroras, but at the same time it lightens up the surrounding landscape which is very special. New Moon reveals most details of auroras and there will be plenty of stars in the sky. First and last quarter (half the Moon visible) are a good mix of everything.

Here some examples of different moon light:

Autumn auroras in moonlight
Snow-covered trees in moonlight
Auroras without moonlight