The best time to visit Cascais, Portugal, is during the late spring (May to June) or early autumn (September to October). During these shoulder seasons, the weather is pleasantly warm with temperatures ranging from 20°C to 25°C, making it ideal for both beach lounging and exploring the nearby Sintra mountains. July and August are the peak summer months; while the weather is hot and sunny, the town becomes extremely crowded with tourists, and hotel prices reach their highest point. For those interested in surfing, the winter months (November to February) offer the best swells at Guincho Beach, though the town itself is much quieter and cooler. A high-value peer tip is to visit in mid-June, when you can experience the "Santo António" festivals in nearby Lisbon, providing a vibrant cultural atmosphere with street parties and traditional grilled sardines, all while enjoying the more relaxed seaside vibe of Cascais during the day.