In general, when booking your flight to Chicago, February is the cheapest month on average.
On average, from most locations, when planning your trip to Chicago, try to book your flight in the region of 32 days in advance for the cheapest prices.
From most locations, the cheapest day to fly to Chicago is Tuesday. Most users found that departing on a Friday was most expensive.
Typically, cheap prices to Chicago are most often found for midday flights. Cheap prices are less likely for afternoon flights.