To be honest, there is a whole heck of a lot more to do in this city in the warmer months.
However, Montreal has excellent opera. I have seen many a show at L'Opera de Montreal so if that is something you like, I'd recommend it.
Often when I travel, I check
www.pollstar.com and check the city and the dates I will be there to see if there are any interesting concerts in town during my trip. I also check to see if there are any pro sporting events scheduled during my visit too, and book the tickets ahead of time if necessary.