June, September, and October are mild and sunny. November through March the temperature ranges from crisp to bitter, April and May can fluctuate between cold/soggy and bright/warm, and July and August can either be perfect or serve up the deadly combo of high heat and high humidity. That said, the only thing certain about Chicago's weather, according to locals, is that it can change in an instant. If you head to Chicago in warmer months, you'll be able to catch some of the fantastic outdoor festivals; during the holiday season the city's decked out in lights.