Every day is poutine fest in Montreal if you want it bad enough.
The big one I've always wanted to check out is Poutine Fest in nearby Drummondville. Why Drummondville? Many towns in Quebec lay claim to the originator of the poutine, but Drummondville has the most evidence to back it up. Good music and beer there too.
Montreal also has something called Poutine Week, which is more decentralized. Dozens of restaurants participate, offering their own take the dish.
Anyways, Poutine Fest in August in Montreal is a good time, but I will caution the lineups can get pretty long and if you don't go on the first day stuff will start to run out.
If ever you make your way here, my Top 6 poutines are as followed:
1. AA (provided the gravy is fresh, check for the pot being stirred)
2. Chez Claudette
3. Lobster poutine at Garde Manger if you want it fancy
4. Chez Tousignant
5. Orange Julep (my wife's number one by far)
6. Piri Piri chicken one at Ma Poule Mouillee