We live in Minnesota and have seen spectacular northern lights at all times of the year. The best I ever saw was in northern Minn in August, almost like fireworks in intensity. The next best sighting for us was in Wisconsin one September night in the northern woods. As was said earlier, the key is to be away from lights and hope there has been lots of solar activity.
We'll be looking for them in July from the Coral Princess too. Always in awe of them!