Theory: Saariselka is a good place for chasing the Aurora as it is located above the Arctic Circle and in the wilderness. The Northen Lights are visible in Lapland from late August till late April. If sky is clear (no clouds), Solar and magnetic activity is high, no full moon - chances to see the Lights are promising.
Practice: Northern Light trip to Lapland is like a casino. No one can guarantee the Northern Lights as it is natural phenomenon.