Spring (April–May) is the best time to visit Utrecht. The weather is mild (12–18°C), the terraces along the Oudegracht open up, and the city is at its liveliest. The Trajectum Lumen light art trail is best experienced in the longer evenings of late spring.
Summer is warm (20–25°C) and busy — the student population largely disappears, but tourists arrive. Outdoor events fill the squares and the canal wharves become crowded terrace bars. July's Festival aan de Werf is the main summer arts event.
Autumn is underrated: the crowds thin, the university comes back to life in September, and the canal trees turn golden. Winter is quiet but atmospheric — cold and sometimes grey, but the brown cafes are at their most inviting and there is rarely a queue for the Dom Tower.
The spring fish migration (March–May) is when the Fish Doorbell at the Weerdsluis is active — a peculiarly Utrecht pleasure worth timing a visit around.