I would say bring a raincoat and umbrella. We still have wonderfully high temps here in Toronto, but we will get rain tomorrow, and temps drop gradually by Thursday. Same for Montreal. Won't be terribly cold, at least not while you're in Montreal, just wet. Environment Canada is a good source for weather reports of specific cities:
http://www.weatheroffice.gc.ca/canada_e.html

On another topic, this Air Canada flight attendant strike looks very real, scheduled for Thursday morning. If I were flying in on Thursday on AC or a codeshare, I would be trying to find another way to get to Montreal. The federal government has threatened to order them back to work immediately, but in my experience, this process takes a few days, so I wouldn't count on it.

Of course, the flight attendants might settle their dispute before Thursday.

Edited to add: this morning govt claims they would pass back to work legislation "immediately". Not sure what means, but it won't be before the strike starts, which unfortunately is Thuresday.