Naples in the Campania region of southern Italy, exhibits a typical Mediterranean climate. It's hot in Summer; cool and damp in the winter. It can be foggy along the coast in winter, although there will also be bright, sunny days which will be tolerable due to the usually mild evening temperatures.
Avoid July and August if you can--Italians do. Notice not only the highs on the temperature chart, but also notice that the night temperatures in summer don't cool off all that much. In a hotel you run the risk of sweltering without air conditioning. You're better off renting a house or apartment that you can get an airflow through.
The best time to visit is April-June and September-October, when temperatures are the most pleasant. Lots of visitors will choose December for the Christmas festivities. Bring a warm overcoat to ward off the occasional icy wind.
Naples, Italy Travel Essentials
Amalfi Coast Map and Visitor Information
Campania Map and Visitor Information
Naples Hotels - User Rated
Guided Tours of Naples, Pompeii, and the Amalfi Coast