The most pleasant times to visit Italian cities are the late spring and early autumn, before the weather gets too hot and uncomfortable, however, if your holiday takes in Tuscany or the Italian Lake District, then summer is also a good time to go, while beach destinations such as Rimini are popular throughout the year.
Somewhat predictably, Rome gets extremely busy over Easter and Christmas, as the faithful from all over the world descend on the Vatican City in order to hear the pope addressing them, and the winter also attracts pilgrims of an entirely different sort to the Italian Alps for the excellent skiing on offer.