Naples is best in spring (April–June) and autumn (September–October), when temperatures are moderate and the crowds thinner than in the height of summer. July and August are very hot and humid; the city partly empties as locals leave for the coast, but tourist numbers remain high and queues at major sites are long.
The Feast of San Gennaro (19 September) is the city's most important religious festival, when the faithful crowd into the Duomo to watch the liquefaction of the saint's dried blood — a miracle the Neapolitans take seriously. It is not a tourist spectacle, but you can attend if you arrive early enough.
December brings the presepe tradition to life in the workshops of Via San Gregorio Armeno. Christmas in Naples has a particularly strong character — the nativity scenes displayed in churches around the city are worth a detour even if you are not religious.
Winter (November–March) is rainy and cold, but Naples is never really empty. Hotels are cheaper and sites quieter, and the city's museums and restaurants keep the same quality year-round.