Canada has 9 festivals in this guide, spread across 7 cities. They run in January, June, July and August, and festival-day temperatures range from -8°C (18°F) to 31°C (88°F), with nights down to -16°C (3°F). That range is the reason there is no single "Canada festival outfit". The answer changes by month and by site.
2 of them run in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment. 1 sits below 16°C, where the outer layer effectively is the outfit.
The ground matters as much as the sky. You will find snow, city streets, forest and city park, which puts insulated waterproof snow boots at the top of the list for most of these events. 2 of the 9 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans rave, edm, neon and streetwear. Calgary Stampede is the largest of them at around 1,200,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.