I'm really really really surprised that not one person has rated the Grand Prix yet.
Montreal's annual (minus the short withdrawal from F1) F1 race is a 3 day event were people come to this fun and beautiful city to not only experience some fantastic races but to enjoy the city. The Grand Prix has become more than just a race...it's become a multi-day event showing off the lovely city of Montreal.
You can purchase grand stand tickets for different sections of the track which is assigned seating as well as general admission which is standing room around the track. It is wise to purchase ear plugs if you're going to be standing beside the track as the cars get to some crazy piercing sound like a jet engine! You can purchase tickets for just race day or all the days which include qualifying.
The city is extremely full and extremely lively during this weekend. Most of the hotel rates I've seen are triple the price this weekend in Montreal, so you will be gouged there! Be warned, you do not want to be driving or taking a taxi during this time. You will not get far in a very short period of time. It can be frustrating.
The city has parties and events going on all weekend. There are booths setup throughout the city that weekend promoting all kinds of things and people walking around giving out free samples. It's a weekend that's definitely worth checking out and possibly the largest annual event in Canada along with Canada Day in Ottawa and Caribana in Toronto.