We just updated the odds of a white Christmas in Illinois, based on 1981-2010 data. We created the map using stations that had at least 20 years of snowfall data during the 1981-2010 period. A year with a white Christmas was defined as having measurable snow on the ground on December 25.
There can be large differences from site to site. Snowfall is notoriously difficult to measures with two nearby sites having different results due to exposure to sun and wind. In general, the odds are about 40-50 percent in the northern third of Illinois, 20-40 percent in central Illinois, and 0-20 percent in southern Illinois.