Here are the median dates of the first measurable snowfall of the season in Illinois. This map is based on 1971-2000 data. While it is not based on the current 1981-2010 averages, the map is still relevant for the purpose of getting an idea of the dates. Measurable snowfall means at least a tenth of an inch.
In the northern third of Illinois, the first snowfall usually occurs around Thanksgiving. The dates move from November to December once you reach central Illinois (just north of a line between Quincy and Champaign). By the time you reach Carbondale, the date can be as late as December 20.
In about 10 percent of the cases, the first measurable snow occurred as early as November 5 in northern Illinois to November 20 in far southern Illinois.
In about 10 percent of the cases, the first measurable snow occurred as late as December 20 in northern Illinois. South of a line from Quincy to Champaign (Interstate 72), it can occur after the New Year. From Carbondale southward, Illinois, it can be January 20th before the first snow arrives.