The best time to visit Belleville, Illinois, is during the spring (late April to early June) and autumn (September to October) months.During these periods, the weather is generally mild and pleasant, with comfortable temperatures superb for exploring the city's outdoor attractions and attending local events. Spring brings blooming flowers and a refreshing coolness, while autumn offers colourful foliage and crisp air.For those planning to enjoy the city's various festivals, autumn is particularly appealing. Belleville hosts its annual Art on the Square event in May, which is a major draw. The Althoff Catholic High School Oktoberfest, a well-known local tradition, takes place in September, offering a lively experience with food, music, and community spirit.Summer (July and August) can be quite warm and humid, which might be less comfortable for extended outdoor activities.