The best time to visit Rome is during the Spring (April-May) and Fall (September-October) when temperatures are relatively mild. Peak tourist season begins just before Easter (when most tourists visit Rome) and runs thru’ October. June to early September is usually the most expensive and crowded time to visit Rome.
Summers can be very hot, humid and crowded. Many of the locals go on vacation in August (especially during the last two weeks), so expect some of the hotels, restaurants and shops to be closed. Winters can be cool and wet. Visitors may be able to get good deals during this time (except during the holiday season) and enjoy the city without the throngs of tourists.