They are strongest in September/October and then again in March, but you can see them any time of the year as long as it is dark enough (typically late August to early April near the Arctic Circle). The main problem is the weather. Autumn (Sept) and into early winter (Dec) is typically cloudy and wet, while it gets drier and less cloudy during winter and early spring (Jan-Mar). That said, this is not a pattern to rely 100% on, but it gives you a clue about what to expect. This is for the western coast of Northern Norway (where this picture was taken). Eastern parts (Finnmark) and the interior of Northern Sweden and Finland tends to be drier and less cloudy and with a more stable winter.