The Italian capital is a year-round city. There’s no best time to visit Rome, and you can really plan a trip at any time of year.
If you can though, avoid visiting in July and August, when the weather is hot and sticky, and those Romans who don’t make their living exclusively from the tourist industry have left town; many businesses close in August.
The weather is more pleasant in May, June and September, when days will be warm but not unbearably so; April, outside Easter, and October, are quieter and the weather can still be clement.
The winter months can also be nice, with many of the city’s more popular sights relatively uncrowded: there may be rain but temperatures are usually mild.