Winter in Montreal is not kind, it is absolutely freezing, making the city a great choice for a starting point for skiing trips in the nearby mountains, although to get around the icy temperatures, the city has an underground area of 29KM of shops and restaurants which is climate controlled in order to make a visit less painful.
Summer is long and hot, with plenty going on, especially in June, when Montreal hosts a Grand Prix, and also a Jazz festival.
The autumn fall of leaves in the rural areas not far from the city is a pleasant time to visit, although guesthouses in the countryside do put up their rates quite a lot.