Douro River Cruise in January

April through June and September through October hit the sweet spot. May brings wildflowers cascading down the terraces and perfect 70-degree weather. September is harvest season—you might catch grape-picking at the quintas, and the valley buzzes with activity. October offers the most dramatic scenery as the vine leaves turn golden against the dark schist. July and August get crowded and hot, with temperatures pushing 90 degrees and the valley feeling more tourist-heavy. Winter cruises run December through February but expect rain and many quintas close for the season. The locks at Carrapatelo—Europe's highest—sometimes close for maintenance in January, limiting itineraries. Book harvest season cruises by March; they sell out first.