Well, it depends on how cold you get: I am comfortable with a long-sleeved t-shirt of some thickness, but you have people spending the whole show with their jacket on. Do you get cold easily?
It's warmer than outside at this time of year (in Québec city), I'd guess. Though I'm no good with numbers, I'll guess it's around 15-18C.
In any case, you'll need a jacket for outside, so if you get cold inside the arena, you can always put it on.