Rotterdam works well year-round, but summer (June–August) is the best time for the waterfront, the terraces, and the open-air markets. Temperatures reach 22–25°C and the Nieuwe Maas comes alive with boats. The North Sea Jazz Festival in July — one of Europe's largest jazz events — brings world-class performers to the Ahoy Rotterdam arena.
The International Film Festival Rotterdam (IFFR) in January–February is the city's most important cultural event: ten days of independent and avant-garde cinema, screenings across multiple venues, and an atmosphere that warms the darkest month of the year.
Spring (April–May) is good for architecture walks — mild weather, long light, and the gardens of the Museumpark in bloom. Autumn is underrated: the summer crowds have gone, the museum season kicks into gear, and the bars are at their most inviting.
Rotterdam's weather is classic maritime Dutch: mild, changeable, and prone to rain at any season. Pack waterproofs regardless of when you come.