Summer (late June to Labour Day in September) is peak season — the lakes are warm enough to swim, the trails are open, and Algonquin is at its busiest. Accommodation in and around Huntsville is expensive in July and August and needs to be booked well in advance.
Late September and October are arguably the best time to visit. The autumn colours are spectacular, the crowds thin out, and the temperatures remain comfortable for hiking and canoeing. The Algonquin colours typically peak in early to mid-October.
Winter has its own appeal: Arrowhead Provincial Park's skating and ski trail draws visitors from Toronto on winter weekends, and snowmobiling is serious business in the area. Spring (April to early June) is mud season — the trails are soggy, the black flies are fierce, and most cottage businesses are closed.