Every year the amusement park La Ronde of Montreal organizes a series of firework shows launched from Jean-Drapeau Park. Firework manufacturers from all over the world compete in 30-minute events on Saturday nights at 10PM during the summer. The calendar of the shows is available here.
The best spot to enjoy the fireworks is from the Clock Tower at the Old Port of Montreal: it is free and the view from the Port is just perfect. You can get there easily with public transport: buses and the subway all get there. If you want a hint, get there quite early: you can take the occasion to enjoy the streets of Old Montreal.
You can also stroll at the Terrasses Bonsecours, just next to the Grand Roue of Montreal. At sunset, you will enjoy a romantic scenery over the Saint Lawrence river.
However, as the event is free, keep in mind that it can get very crowded. If you want to enjoy the fireworks comfortably sitting in a long-chaise, drinks and snacks, you might want to invest 5 CA$ for a spot at the Beach of the Tower Clock.
As the sun goes down, the city lights will be your only point of reference, setting the scene for the firework show. One more hint: bring a blanket or a warm jacket. The breeze from the river can get very chilly!
Enjoy your summer night!