Known as the Peshtigo Fire, it occurred on October 8, 1871, the same day as the Great Chicago Fire. The Peshtigo Fire was much deadlier than the Chicago Fire and it actually holds the record for the deadliest fire in US history.
It occurred in Peshtigo, Wisconsin, which is relatively close to Chicago, IL. Estimates put the death toll at somewhere between 1200 and 2500 deaths. The total area affected by the fire was around 1875 square miles, or twice the size of Rhode Island.
October 8th was actually a pretty active day for fires. Across Lake Michigan, the cities of Holland and Mainstee, Michigan were also affected by fires, as well as the city of Port Huron, on the southern end of Lake Huron. Some have speculated that something like a meteorite could've caused this, but scientists believe that is probably not true.