Seville's climate is famously extreme. Summer (June to September) is brutally hot — temperatures regularly exceed 40°C (104°F), and even locals flee to the coast in August. Avoid these months if you can; if you must visit, stay in the shade between noon and 6 pm and save sightseeing for the early morning and the late evening. Winter (December to February) is mild (daytime highs around 15°C), brief, and pleasantly empty.
The two best times to come are the twin spring festivals that define the Sevillian year. Semana Santa — Holy Week — runs from Palm Sunday to Easter Sunday, and the streets fill with hooded penitents and enormous baroque floats carrying images of Christ and the Virgin in the most theatrical religious event in Europe. Two weeks later comes the Feria de Abril, a week-long spring fair with more than a thousand striped tents set up on a dedicated fairground by the river, where locals drink rebujito (chilled manzanilla with 7-Up), eat tapas, ride horses, and dance sevillanas until dawn. Both are unforgettable; both also mean hotel prices double and rooms book out months in advance.