Summer, June to August
Warm and humid — 25 to 30°C typically, with humidex readings that make it feel considerably hotter, and periodic heat warnings.
This is festival season: Pride in June, Caribana at the start of August, Taste of the Danforth, patios everywhere, and the islands and beaches in use.
It is busy and prices are high. Air conditioning is not optional.
Autumn, September to November
The best time to come. September is warm without the humidity, October is crisp and clear, and the autumn colour in the ravines and out in the Muskokas is a genuine event.
TIFF in September fills the hotels and is either the reason to come or the reason to avoid the month.
November is grey and the leaves have gone.
Winter, December to March
Cold — around minus 1 to minus 10, with snaps below minus 20 with wind chill — and around 120 centimetres of snow across the season.
It is also the cheapest time by a wide margin, and the city is organised for it: the PATH, the skating rinks, the Christmas Market at the Distillery, Winterlicious in late January. See winter in Toronto.
The museums are the point in February, not a fallback.
Spring, April to May
The weakest season. March and April are grey, wet, and produce slush that refreezes. Torontonians do not enjoy it.
It turns quickly, usually in late April, and the High Park cherry blossom follows within days — about a week of bloom, and the park closes to cars for it. The city runs a bloom tracker.
May is genuinely pleasant and much quieter than June.
Dates worth knowing
Victoria Day, the Monday before 25 May — the unofficial start of summer, with fireworks. Canada Day, 1 July. Caribana, the August long weekend — the parade is the Saturday. TIFF, ten days in September. Nuit Blanche, one Saturday in early October, all night and free. Thanksgiving, the second Monday in October — Canadian, not American. The Santa Claus Parade, mid-November, running since 1905.
The humidex and the wind chill
Environment Canada publishes both, and they matter more than the raw temperature. A 30°C day with a humidex of 40 is genuinely oppressive; a minus 8 day with a wind chill of minus 20 freezes exposed skin.