The Northern Lights, also known as Aurora Borealis, can appear any night when it's enough dark outside. However, in order to have the best chance of seeing them, you should plan to visit during the winter months when the nights are longer and darker. The peak season for viewing them is typically from September through March.
Additionally, according to NASA's Aurora tracker, on a clear night during winter it’s best to look for the Northern Lights between 10 pm and 2 am. That's because this is when the Earth's field lines are most aligned with the Sun and thus when you'll get maximum solar activity.
Finally, if you are a light sleeper or hoping to catch a glimpse while you dream, set an alarm for midnight so that you don't miss out! With a bit of luck and patience, these natural wonders will truly take your breath away.