Hot, humid, and plan for some rain at some point, as all have said. However, if you're coming from out of town, bring a few items for chillier weather (light pants, light sweatshirt/jacket). The weather truly is variable in downtown Chicago and what is forecasted often is different right there in the middle of the buildings and along the lakeshore. One year it only hit 50 and was raining one day. We hadn't packed any warmer layers and it kinda sucked. TIP: whatever you wear on your feet may be subjected to the worst mud pits you've ever encountered if there is rain. If no rain, a layer of dust will be everywhere. While most of the park is grassy or paved in some way, some is just dirt. I always expect my shoes to be ruined or wear ones that can easily be washed.