Valencia is comfortably visitable in any month — winter lows rarely drop below 6°C, summer highs are usually around 30°C, and the city sees 300 sunny days a year. The shoulder seasons, April–June and September–October, are the sweet spots: warm enough for the beach, not yet boiling, and not so packed with tourists.
The unmissable date is Las Fallas, 15–19 March. The city builds 700 enormous papier-mâché figures (the fallas) for a year, displays them for a week, sets off the daily Mascletà pyrotechnic show at 2 pm in the Plaza del Ayuntamiento, and on the night of 19 March burns every figure to the ground in a single fiery night called La Cremà. It is louder, more chaotic, and more cathartic than almost anything else in Europe. Book accommodation months ahead.
August is hot, humid, and half the city is on holiday — many of the best restaurants close for weeks. Avoid unless you want lazy beach days. The July Feria and the October feria (the city's regional holiday) are smaller but still festive.