April to June and late September to October are the best months for traveling in Italy -- temperatures are usually mild and the crowds aren't quite so intense. Starting in mid-June, the summer rush really picks up, and from July to mid-September the country teems with visitors. August is the worst month: Not only does it get uncomfortably hot, muggy, and crowded, but the entire country goes on vacation at least from August 15 to the end of the month -- and many Italians take off the entire month. Many hotels, restaurants, and shops are closed (except at the spas, beaches, and islands, which are where 70% of the Italians head). From late October to Easter, most attractions go on shorter winter hours or are closed for renovation. Many hotels and restaurants take a month or two off between November and February, spa and beach destinations become padlocked ghost towns, and it can get much colder than you'd expect (it might even snow).
It's warm all over Italy in summer; it can be very hot in Rome. The high temperatures begin in Rome in May, often lasting until sometime in October. In Rome the weather is warm year-round, averaging 50°F (10°C) in winter.
For the most part, it's drier in Italy than in North America, so high temperatures don't seem as bad because the humidity is lower. In Rome, temperatures can stay in the 90s for days, but nights are usually cooler and more comfortable.
The average high temperatures in central Italy and Rome are 82°F (28°C) in June, 87°F (31°C) in July, and 86°F (30°C) in August; the average lows are 63°F (17°C) in June and 67°F (19°C) in July and August.