Lisbon is a year-round city, but the peak tourism season runs June to September when the city is hot, crowded, and expensive. July and August are the busiest months; prices are highest and the old-city neighbourhoods, particularly the Alfama and Belém, can feel overcrowded.
May and June are excellent: warm, flowers blooming on the jacaranda trees along Avenida da Liberdade (which turn the whole street purple in late May), and the Santo António festival in June when the Alfama celebrates with street grilling, sardines, and dancing. October and early November offer warm days, thin crowds, and beautiful light.
Winter (December to February) is mild by northern European standards — temperatures rarely drop below 8°C — and the city has a pleasant unhurried quality. The museums, restaurants, and cafés are all open, and accommodation is cheaper. Rain arrives in November and continues intermittently through winter; bring a waterproof.