Spa is a year-round destination, though each season has a different character. Spring and autumn are ideal for walking — the forests are at their most beautiful, and the town is quiet. Summer brings the Formula 1 crowds in late August (book accommodation very far in advance if you plan to be here for the Belgian Grand Prix), and the Ardennes fills with cyclists and hikers. Winter is cold and often grey, but the thermal baths are at their most appealing when it is freezing outside, and the Hautes Fagnes plateau sometimes gets proper snow.
Rainfall is spread throughout the year — the Ardennes is a wet region, and Spa itself is notably rainy. Bring layers and waterproofs regardless of season.