May through September is the comfortable window: long days, café terraces on the Grote Markt, and outdoor concerts at the Noorderzon arts festival in late August. Winters are dark and damp rather than bitterly cold, but the city compensates with cosy brown cafés and the Eurosonic Noorderslag music festival in mid-January, which fills the centre with bands from across Europe.
September is the best month for visiting if you want to feel the student energy — the academic year starts and the streets refill. Avoid the dead weeks between Christmas and mid-January when the university empties out.