Summer (June–August) is the obvious choice — the hiking season in Sleeping Giant is at its best, the waterfront is active, and temperatures are mild (highs around 25°C). July and August are the busiest months; book Sleeping Giant campsites months ahead.
Winter is severe — temperatures regularly drop below -20°C, and the surrounding lake effect can bring heavy snow. But the city is functional year-round, and there is decent cross-country skiing at Sleeping Giant and downhill skiing at Loch Lomond, just east of the city. Spring and autumn are transitional; the fall colours along the Superior shoreline in September and October are worth the trip on their own.