Rome has a nice weather and this makes the eternal city visitable year-round; however, spring and autumn is without doubt the best times to visit, with generally clear skies and mild temperatures (even if late autumn, November, can be rainy). July and August are humid and hot (many Romans desert the city in August so many businesses close at this time); from December to February there is briskly cold weather, although it's rarely grey and gloomy.
In Rome the toursit season starts generally from the first of April to October and again for the two weeks around Christhmas and new year's Eve.
August is a very nice month in Rome, because you can find the city empty and you can discover the treasures of the eternal city, like the vatican and the ancient roman ruins.
During the Holy Week (Easter) Catholics from around the world make pilgrimages to Rome's various basilicas and to hear the Pope give his blessing from the window of its apartment in sixty languages. On Good Friday there is the pope's procession of the Cross from the Colosseum to Capitoline Hill that is worldwide broadcasted: you do not need tickets to attend it.
Making a reservation at the hotels of Rome during easter period is really very hard and you can have the unpleasant surprise to change your hotel accommodation very often due to the overbooking system.
