Summer is the best time to visit Chicago. The weather gets hot and humid -- regularly into the mid 30s --but temperatures are moderated somewhat by Lake Michigan. It's usually sunny too, though severe storms can pop up suddenly. Summer is also filled with festivals and cultural activities.
Temperatures are more comfortable in spring and fall, though the weather can change on a dime as hot and cold air masses duke it out. Winters are cold and snowy, but the holiday season (November and December) can be a magical time. There are plenty of festivals, and the city glows in the holiday lights. Chicago receives over 100cm of snow each year, which can make getting around difficult.