The best time to visit Cornwall, Ontario is during the summer months, from June to August, when the weather is warm and sunny, making it ideal for outdoor activities.During this period, the St. Lawrence River is great for boating, fishing, and kayaking. The Waterfront Trail, which stretches along the river, is excellent for cycling and walking, offering scenic views and access to various parks. Festivals and outdoor events are also prevalent throughout the summer, providing entertainment and a chance to experience the community's spirit.Spring (April to May) and Fall (September to October) are also pleasant times to visit. The temperatures are milder, and the natural scenery is particularly appealing, with blooming flowers in spring and colorful foliage in fall.