Cascais has the most reliable weather on the Atlantic coast of Portugal — about 260 days of sun a year, and noticeably cooler than Lisbon in summer thanks to the Atlantic breeze. The town is at its best in May, June, September and early October: warm enough to swim (the water hovers around 18–20°C in summer), uncrowded, and bright until late evening.
July and August bring the heaviest crowds — many Lisbon residents have summer houses here, and the seafront cafés and beaches fill up. Winter is mild (daytime temperatures usually 12–16°C) and rarely freezing, but it does rain. The annual Cool Jazz Festival in July is a high point, and the Cascais Christmas Village transforms the centre with lights and a market through December.