Here are the festivals and events that happen each May in Rome. Note that May 1, Labor Day, is a national holiday, so many businesses, including most museums and some restaurants, will be closed.
May 1 - Labor Day
Primo Maggio is a national holiday in Italy, so many Romans either head out of town or stick around for the huge concert in Piazza San Giovanni, usually starting in the early afternoon and continuing until around midnight.
There are often protest rallies as well which may cause local transportation disruptions. Most sites and museums are closed but you can still walk on Via Appia Antica where a couple of the catacombs are usually open or visit the ancient Roman site of Ostia Antica, a short distance from Rome. Of course, open-air sites like Piazza Navona and the Trevi Fountain are always open.
First weekend in May - Open House Roma
Guided tours of buildings and architecture studios in Rome. Free but reservations required through Open House Roma.
May 6 - New Vatican Guard
A new group of Swiss Guards is sworn in at the Vatican every May 6, the date that marks the sack of Rome in 1506. The guards are sworn in in a ceremony in the San Damaso courtyard within the Vatican City. The general public is not invited to this ceremony, but a glimpse of the swearing-in may be possible if you are booked for a private guided tour of the Vatican on that day.
Early- to Mid-May - Italian Open Tennis Tournament
Rome hosts the Internazionali BNL d'Italia, also known as the Italian Open, each May at the tennis courts at Stadio Olimpico. This nine-day, clay court event is the largest tennis tournament to precede the French Open, so many major tennis stars use the Italian Open as a warm-up.
Mid-May - Museums Night
This yearly event takes place in many European cities. Museums are open at night with special events and free admission, usually starting at 8PM. See La Notte dei Musei.
T here's a lot to do in Rome in June, too.