Spring and fall are the best times to visit, with mild temperatures and many sunny days. Summers are often sweltering. In July and August, come if you like, but learn to do as the Romans do—get up and out early, seek refuge from the afternoon heat, resume activities in early evening, and stay up late to enjoy the nighttime breeze.
Come August, many shops and restaurants close as locals head out for vacation. Remember that air-conditioning is still a relatively rare phenomenon in this city. Roman winters are relatively mild, with persistent rainy spells.