Statistically, springtime (March/April) and autumn (September/October) contain the most "geomagnetically disturbed days" which means more aurora activity...although you're bound to see the lights in October, November, December, January and February if you spend enough time looking up.
In Yukon, it's dark enough to see the northern lights from about August-through-April. During May, June & July it's just too bright in to see them in the "Land of the Midnight Sun."
