The right season makes a city. The wrong season ruins it. Here is our cheat sheet for North America.
Spring (March–May): Washington DC for the cherry blossoms. Charleston and Savannah before the humidity kicks in. New Orleans for Jazz Fest. Mexico City year-round but particularly perfect in spring.
Summer (June–August): Vancouver, Seattle and Portland are at their best. Montreal’s outdoor festival season is unmatched. Toronto is alive on patios. Avoid Phoenix and Las Vegas. Avoid the South generally.
Fall (September–November): New York at its peak. Boston for the foliage. Vermont and Quebec City for the leaves. San Francisco enjoys its only good weather of the year.
Winter (December–February): Miami, New Orleans, San Diego and Tucson. Mexico City still works. Avoid Chicago, Toronto, Montreal and the Northeast generally unless you genuinely love winter.
The pattern: most American cities have a 3–4 month sweet spot and are difficult to visit outside it. Plan accordingly.