The weather in Rome is a great place to start your planning. Blessed with a truly Mediterranean climate, for most of the year you can't really go wrong with a visit. However, there are subtle differences to the seasons that can make all the difference so that you can enjoy Rome from a day spent shopping to pre-dinner drinks.
From May through August the temperatures heat up, as summer in Rome goes into full swing.
This is when it's handy to keep a water bottle with you and take advantage of the many nasone (drinking fountains) that pour fresh, cool drinking water at all times. Summer is also a great time to explore the many lake and beach towns of Lazio where you can relax on the beach with Romans and enjoy the outdoor restaurants and bars that dot the coastline.
Early September still has the lingering effects of summer, but things do cool down and the crowds begin to lessen in the city as Rome moves toward the Fall.
High temperatures are often still in the 20 degree Celsius (70 degree Fahrenheit) range, making this a pleasant time to visit the city. The same can be said for March and April, though the Easter holiday can make the city more crowded at that time of the year.
The coolest, and rainiest times of the year in Rome are November through March.
If the weather in Rome at this time of the year doesn't bother you, it can also be a wonderful time to visit the city without the larger tourist crowds.