It will be COLD during the primetimes, early and late in the day!
I would suggest layers, but most importantly is a quality Parka and Bibs. I suggest a quiet, wind and water proof system. My favorite is the Whitetail Extreme MTO50 series from Cabela's..... a little expensive but well worth it for the all day comfort.
I'd also suggest a good neck gaitor to keep your heat in and a qualit pair of pac boots. Based on what many non-resident friends have stated when they've come up to hunt..... the all day cold can be hard to deal with when you don't have the right gear.
Best of luck on your Illinois hunt.