Summer (June through August) is peak season and for good reason -- the city comes alive with outdoor festivals, beach weather, and long evenings on restaurant patios. Temperatures reach the upper 70s to low 90s Fahrenheit, and thunderstorms are common but usually brief.
Early fall (September and October) is arguably the best time to visit. The crowds thin out, the weather is comfortable, and the trees along the lakefront turn color. Spring is pleasant but unpredictable -- a 70-degree day in April can be followed by snow. Winter is harsh, with temperatures regularly below freezing from December through February and an average of 38 inches of snowfall. But winter has its own appeal: the museums are quiet, holiday events fill the calendar, and there is something bracing about walking the Magnificent Mile in a proper Chicago wind.