Rovaniemi lies within the auroral oval, the zone around the magnetic pole where the northern lights appear most frequently and most intensely. From late August through April, on clear nights with sufficient solar activity, the lights can appear overhead in waves of green, occasionally violet or red. The show ranges from a faint greenish smear to curtains of colour that fill the entire sky and move fast enough to track in real time.
The practical approach is simple: stay at least three nights (to improve the odds against cloud cover), get away from the city lights, and watch the space weather forecast at space.fmi.fi or through the Aurora Service app. The forests north of the city and the lakeshores around Rovaniemi offer dark sky viewing within easy reach. Most tour operators in Rovaniemi run northern lights snowmobile or snowshoe trips that take you into dark areas at the most likely hours — typically 9pm to 2am. The experience is deeply dependent on luck and weather, which is part of what makes it memorable.