The best time to visit Sullivan is during the warmer months, roughly from late spring to early autumn, particularly between May and October.
During this period, the weather is generally pleasant, making it ideal for outdoor activities and exploring the local area. You'll find comfortable temperatures for walking around the downtown area, visiting Sullivan City Park, or enjoying the nearby Shakamak State Park.
For those interested in local events, late spring and summer often feature various community gatherings and festivals, providing a good opportunity to experience the local culture. The Sullivan County 4-H Fair, for example, typically happens in late June or early July and is a popular attraction.
If you prefer cooler temperatures and fewer crowds, early autumn, specifically September and October, can also be a lovely time to visit.