The main tourist season starts at Easter and runs until October; peak periods are in spring (April, May, June) and autumn (September and October, It's famous the Ottobrata Romana).
Numerous outdoor festivals and concerts (Villa Celimontana, Estate Romana ext) and the fact that Romans desert the city for the beaches and mountains, which means very light traffic and a less-crowded city centre, makes summer almost worth the heat.
If you do visit in summer, try to hit the sights early, take a long lunch and a nap, and then head out again around 18:00 to take advantage of the cooler evening.

Rome Hotels Full List
Full List Winters are usually mild with few tourists and some fun events around Christmas time. I't's hard to make a selection of sights as most of Rome's sights should not be missed.
Anyway we suggest you the ones we think could leave you the strongest emotions during your stay in Rome; Colosseum and the Roman Forums, St.Peter and the other Basilicas, Spanish Steps and Trevi Fountain are some examples of the wonderful Rome sights.

Rome's mild climate makes it visitable year-round; however, spring and autumn are without doubt the best times to visit, with generally sunny skies and mild temperatures (although late autumn, November, can be rainy). July and August are unpleasantly 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.