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."
No! It is a myth that the northern lights happen only when it's cold. They happen year-round and since there is more darkness to see them in the winter people associate the cold winter with the auroras. August and September is a great time to see the Aurora while wearing a T-shirt and shorts!
It is not mandatory, but we recommend it. December to April, the temperature drop to around -20 to -25℃, so our winter gear helps you to enjoy any tour comfortably and safe. We strongly recommend to wear our rental winter gear during snow shoeing, ice fishing, and Aurora Viewing tours.