The best time to visit Collinsville is spring (April–May) or fall (September–October), when temperatures are mild and the Cahokia Mounds site is at its most pleasant. Summer is hot and humid — Illinois summers can be brutal, with temperatures regularly exceeding 90°F — though the Cahokia Mounds Interpretive Center is air-conditioned and worth visiting regardless. Winter is cold and grey but uncrowded.
Cahokia Mounds World Heritage Festival is held in late September, with cultural performances, Native American vendors, and living history demonstrations at the site. It is the best single day to visit if you want to see the place come alive.
Route 66 Memories events take place periodically through summer, celebrating the city's place on the historic highway with car shows and local festivals.
The Catsup Bottle Birthday Party is held annually in July — a small, cheerful celebration of the World's Largest Catsup Bottle that draws Route 66 enthusiasts and roadside attraction fans from across the region.