Timing matters. In Rovaniemi the Northern Lights are statistically most active between roughly 9pm and 2am, with the sweet spot often around 10pm–midnight — which is exactly when guided tours are out in the field.
The prime window
Geomagnetic activity tends to peak around magnetic midnight. That's why most aurora tours run in the late evening rather than early night.
December is different
Around the winter solstice, Rovaniemi has under four hours of daylight, so the sky is dark from mid-afternoon and the aurora can appear as early as 2–4pm.
Be ready to wait
The aurora comes in waves — a burst, a lull, then another burst. Staying out through the prime window, rather than giving up after 30 minutes, is the key to catching a strong display.
Let a guide track it
On tour, your guide monitors live data and keeps you out during the peak, moving to clear skies as needed.