Although some of Montreal's attractions make a winter visit enticing (especially for skiers and snowboarders), summer is the best season to visit the city. Spring is probably least interesting, because the snow is just melting and some outdoor attractions don't open until late June.
Visitors who go in summer will want to be prepared for the heat (and an occasional thunderstorm). Highs are usually in the low 80s F/25-27 C but occasionally climb into the mid-90s F/32-35 C, with humidity as high as 80%. Use bug repellent when heading out to the countryside in the spring.
Fall temperatures are pleasant, and there's sometimes a late Indian summer. This is the best time to see a symphony of reds and yellows as the fall leaves die out. Average October temperatures run 42-55 F/5-13 C, so take a warm sweater or jacket.
Winter is definitely cold and snowy. When venturing outside, be sure to wear several warm layers and keep your extremities covered. This means a warm jacket, boots, a hat and gloves. Temperatures average 7-23 F/-15 to -5 C in January, with December-March readings possibly below 0 F/-18 C. With wind chill, it can feel like -22 F/-30 C. It snows heavily on occasion, so expect snow banks and snow-packed sidewalks.
Be wary of extremes in temperature fluctuations. One spring morning it may be 61 F/16 C, and by the next day the temperature can fall way below 32 F/0 C. Follow the scouting motto and always be prepared.