Serbia has a continental climate with hot summers and cold winters.
Summer (June-August) brings temperatures of 25-35C. This is festival season -- EXIT in Novi Sad (July) and the Guca Trumpet Festival (August) are two of Europe's most exciting music events. Cities can be very hot, but the mountains offer relief. Belgrade's river clubs (splavovi) are at their liveliest.
Autumn (September-October) is arguably the best time to visit. The heat eases, the light is golden, and the wine harvest begins. The countryside is beautiful, and cultural events pick up in Belgrade. September in particular combines warm days with manageable crowds.
Winter (November-March) is cold, with temperatures often below freezing and occasional snow. Belgrade can be grey but has a cozy charm, with warming rakija in kafanas (traditional tavern-restaurants). The mountain resorts of Kopaonik and Zlatibor offer affordable skiing.
Spring (April-May) is pleasant, with mild temperatures and blooming fruit orchards across the countryside. Easter celebrations, following the Orthodox calendar, can be a fascinating time to visit.
For the best overall experience, aim for May-June or September. For festivals, July and August are unbeatable despite the heat.