Best Time to Visit Civitavecchia and Rome
Between April and June is the best time to visit the Southern half of Italy. The temperatures are comfortable, there's little-to-no rain, and the crowds are small as this is a shoulder season. Additionally, you may be able to find reduced hotel and flight rates during these months.
Between July and September is the high season for this region. It's hot, crowded, and prices are at their height. This is when most people go and it's a fine time to visit Italy, but it probably won't be a super relaxing trip between the high crowds and hot weather.
Between October and March is an okay time to visit Italy. This is the off season and most attractions are on shorter winter hours or may be closed for renovations. That being said, you can get a solid hotel deal during these months, it's not crowded, and the weather only really gets cold (it may snow but it's not likely) between December and February.