Madison has four very distinct seasons, each with its own appeal and its own drawbacks. Summer (June through August) is the most popular time — the Memorial Union Terrace is at its best, the Dane County Farmers' Market and Concerts on the Square are running, and the lakes are warm enough to swim in. The city is lively and outdoor life is the whole point. Expect humid heat, occasional thunderstorms, and the energy of a university town with no classes to attend.
Autumn (September and October) is arguably the best time to visit. The crowds thin out, the lakes are still swimmable in September, and the trees on the UW campus and around the lakes turn a reliable orange and gold. The farmers' market runs through early November.
Winter is brutal by most standards — temperatures regularly fall well below freezing, and snowfall is heavy. But Madison embraces winter rather than hiding from it: ice fishing huts appear on Lake Mendota, there is cross-country skiing in the parks, and the city's indoor café and bar culture is at its most appealing when the weather closes in. The Overture Center's performance season runs through winter.
Spring (April and May) can be cold and wet, but the Arboretum wildflowers appear in April and the Memorial Union Terrace reopens as soon as the weather allows. Graduation season in May brings crowds.