April–May and September–October are the best months: mild temperatures (18–25°C), long days, and crowds that are heavy but manageable. Spring brings wisteria and outdoor terraces; autumn brings the vendemmia and lower prices than summer.
June–August is hot (often above 35°C), humid, and intense with tourists. August is particularly strange — most Romans leave, so the city is simultaneously overcrowded with visitors and emptied of locals. Many family-run restaurants and shops close for two weeks in August. If you go in summer, arrive early at major sights and take a long midday break.
Easter is the most crowded week of the year. The Vatican and centro storico are packed for the entire Holy Week; book accommodation months in advance and expect queues at every sight.
November–March is the quietest and cheapest period. Temperatures are cool (5–15°C) with occasional rain; short winter days reduce sightseeing time. But the museums are far less crowded, the light is beautiful, and hotel rates can drop dramatically. Christmas week is an exception — the city fills up again for the nativity scenes and midnight masses.
Rome's main cultural events tend to fall in May (Notte dei Musei, free museum night), June (Estate Romana outdoor events begin), and October (Rome Film Festival).