Late spring through early autumn is the easy answer. From May to October the patios are open, the islands and the Beaches come alive, and the festival calendar is packed — Pride in June, the Caribbean carnival (Caribana) in late July and early August, the Canadian National Exhibition at summer's end, and the Toronto International Film Festival, which takes over the city each September. September and early October are arguably the best of all: warm days, cooler nights, and the autumn colour in the ravines.
Winter is long and genuinely cold, often well below freezing from December through March, with snow and biting wind off the lake. The upside is a city that embraces it — skating at Nathan Phillips Square and Harbourfront, the Cavalcade of Lights, and indoor refuges from the museums to the PATH. Summer can be hot and humid in July and August. Whenever you come, dress for the season Toronto is actually in, not the one you wish it were.