The season you pick changes which festivals you can actually attend, what packing looks like, and what a realistic daily budget is — here's how to think through it properly.
If a specific event is the reason for the trip — the Quebec Winter Carnival, Just for Laughs, Caribana, the Festival d'été de Québec — the date of that event should anchor the whole itinerary, not the other way around. Building a "four cities in ten days" trip and hoping it happens to line up with a festival is how people miss things by a week.
Winter across all four cities (Vancouver aside) means real cold-weather layering: insulated boots rated for sub-zero temperatures, a proper winter coat, and hand/foot warmers if you'll be outdoors for festival events like Igloofest or the Winter Carnival. Vancouver's winter is the exception — waterproof, not insulated, is the priority there. Summer packing is straightforward across all four, though Montreal and Toronto's humidity means lighter fabrics matter more than in dry Vancouver.
Winter days are genuinely short this far north — expect sunset before 5pm in Toronto, Montreal and Quebec City through December and January, which compresses how much sightseeing fits into a single day. Summer swings the other way, with daylight past 9pm in June and July, which is part of why summer festival schedules can run so late into the evening.
Summer is peak season across all four cities — expect higher accommodation rates, especially anywhere near a specific festival's dates. Winter (outside the Christmas/New Year weeks and specific festival dates like the Winter Carnival) is the cheaper time to visit three of the four cities; Vancouver's mild winter means its own rates don't drop as sharply.
Late September through mid-October (fall) and April through May (spring) fall between this site's winter/summer toggle, and deliberately so — they're genuinely different trips again, generally milder than the extremes on either side, with lower accommodation costs than summer but without winter's festival calendar or Vancouver's dry-summer advantage. If your dates are flexible and none of the specific festivals above matter to you, shoulder season is worth considering on cost alone — just don't expect any of the specific events described on this site to be running.
Pick one season and stay in it — Toronto in January and Vancouver in July on the same trip means packing for both extremes and missing both cities' actual seasonal identity. See Getting Between Cities for how far apart these four actually are before planning to visit all of them on one trip regardless of season.