Leuven is worth visiting year-round, but its character shifts dramatically with the academic calendar. From October to May, the city is full of students and runs at full energy — bars are packed, events are frequent, and the streets have the particular animation of a place that takes pleasure seriously. In July and August, the university empties out and the city becomes quieter and slightly melancholy, though the major sights remain open and the tourist numbers are lower.
Spring (April to June) is the best weather for visiting: mild temperatures, long evenings, and the city still functioning at near-full intensity before the summer exodus. The Leuven Beer Festival, held in the Grote Markt in the first week of May, draws serious Belgian beer enthusiasts from across the country and is worth timing a visit around. Autumn is similarly good, with the academic year resuming in September and the city returning to life.
Winters are damp and cold, but Leuven's centre is compact enough that the distances between warm interiors are short. The Christmas market on the Grote Markt is one of the better ones in Flemish Belgium. Temperatures average around 2–4°C in January and 20–22°C in July; rain is distributed fairly evenly across the year.