There are many annual events in and around Rome. I have listed quite a few below.  It is always a good idea to check with the Rome Chamber of Commerce about the actual dates of those events you would like to see to make sure they are still, in fact, happening and if the date remains the same or has changed. Try writing to Ente Provinciale per il Turismo, via Parigi 11, 00185 Roma (tel. 463.748).   If you are already in Rome and want to visit their office, they are open from 8:15 a.m. to 7:15 p.m. daily. There is another information office at Stazione Termini which is open from 8:30 a.m. to 7 p.m. daily as well (tel. 465.461).  I have also found a wonderful website listing concerts and other events going on at any given time in Rome called whatsonwhen.com and have included it on my Links page for your convenience.

La Notte Bianca (White Night)

Since it started back in 2001, La Notte Bianca (or the White Night) has become a staple in Rome.  One night a year, various museums, shops, theaters, etc. remain open most of the night or all night.  Typically, this event happens sometime in September, but it can change on a yearly basis.  Also, every year, the events change, and the whole schedule of buses, timetables, etc. also change to accommodate this huge event.  So, I have put a link to their official web site (La Notte Bianca) so you can check it out for yourself to see when it will be scheduled for the current year.  The web site is updated about one month before the event takes place, and has the full program, maps, etc.  Not all of Rome's museums take part, but the most important ones indeed stay open (Capitoline Museums, Roman National Museum at Palazzo Massimo, Museum of Rome at Palazzo Braschi, exhibition halls inside the Vittoriano, the Ara Pacis, etc.).  Each of them has a different closing time, ranging from midnight to 6 a.m.  The entry is free of charge, but for this reason there could be long lines (queues).  However, the later it gets, usually the fewer people in the lines (queues).

Spring:

Festa di Santa Francesca Romana (March 9), Santa Francesca Romana. Blessing of the city's cars, buses and trams.

Festa di San Giuseppe (March 19), Trionfale area.  St. Joseph's (and Father's) Day.  Street stalls, food, and music.

Good Friday (March/April - date varies in relation to Easter Sunday)<, Colosseum. Procession of the Cross at 9:00 p.m. led by the pope.

Easter Sunday (March/April - date varies), St. Peter's Square. Address made by the pope.

Rome's Birthday (Sunday before April 21), Piazza dei Campidoglio.

Festa della Primavera (March/April), Piazza di Spagna and Trinita dei Monti. Azaleas in the street and concerts.

Art Exhibition (April/May), Via Margutta.

Maratona Gay/Lesbian Festival (May). Need to contact organizers for dates and place annually: Circolo Mario Mieli di Cultura Omosessuale, or go to bars such as the Hangar (Via in Selci, 69) or, for lesbians, try Panico (Via di Panico, 13). You can also try the most famous gay club in Rome, Alibi, at Via di Monte Testaccio, 44.

International Horse Show (early May), Villa Borghese.

Antiques Fair(Mostra dell'antiquariato) (mid-late May), Via dei Coronari.  Hours 10am-1pm and 4-11pm.

International Tennis Championships (late May), Foro Italico.

Flower Festival (June, the Sunday after Corpus Domini), Genzano, Castelli Romani, south of Rome.  Streets are carpeted with flowers (picture above).

Festa di San Giovanni (June 23-24), Piazza di Porta San Giovanni. Celebrated with meals of snails in tomato sauce, suckling pig, a fair, and fireworks display.

RomaEuropa (late June-late July), mainly in Villa Medici.  Films, dance, theater, and concerts.

Festa di San Pietro (June 29), many churches. Celebrations mark the feast of St. Peter.

Expo Tevere (mid-June/mid-July), along the Tiber between Ponte Castel Sant'Angelo and Ponte Cavour. Mainly local Italian arts and crafts, food and wine (wine often at below-shop prices), olive oil, jams/preserves, music and fireworks.

Festa de Noaltri (last two weeks in July), the streets of Trastevere.  Eating, processions and entertainment.  Many stalls where you can buy almost anything you want.

Arts Outdoors (July/August), Baths of Caracalla, Villa Ada, Ostia Antica, by the Tiber, in parks. Opera, concerts, drama, and film.

Festa della Madonna della Neve (August 5), Santa Maria Maggiore.  Legendary 4th century fall of snow re-enacted with showers of white flower petals.

Ferragosto (August 15), Santa Maria in Trastevere. Midsummer Roman holiday.  Almost everything closes down.  Celebrations are held for the Feast of the Assumption.

Art Fair (September), Via Margutta.

La Notta Bianca (White Night) (mid-September), all over central Rome.  Museums and galleries are open all night and are free.

Sagra dell'Uva (early September), Basilica of Constantine. Harvest festival with grapes selling at bargain prices and lots of folksy street entertainment.

Crafts Fair (last week September/first week October), Via dell'Orso, near Piazza Navona.

Antiques Fair (mid-September/mid-October), Via dei Coronari.  Mon.-Thurs. 3pm-11pm, Fri.-Sun., 10am-11pm.  Selling antiques, jewelry, leather, and other wares.

Festa di Santa Cecilia (November 22), Santa Cecilia in Trastevere and Catacombs of San Callisto.

Vino Novello Tasting (late November), Campo de' Fiori.

Festa della Madonna Immacolata (December 8), Piazza di Spagna.  Firemen climb up a ladder to place a wreath on the statue of the Virgin Mary. Column in front of McDonald's.

Befana (mid-December-mid-January), Piazza Navona.  Christmas and children's market.

Nativity Scenes (mid-December-mid-January), many churches.  Life-sized scene in St. Peter's Square, collection at Santi Cosma e Damiano. Also check out the International Museum of the Christmas Cribs near the Trojan Marketplace ruins. It is amazing!

Midnight Mass (December 24), at most churches.

Christmas Day (Decmeber 25), St. Peter's Square.  Blessing by the pope.

New Year's Eve (December 31), all over Rome.  Fireworks displays, furniture thrown out.