Bucharest has a continental climate with hot summers and cold winters. The best months are May, June, and September — warm but not punishing, the parks at their best, the terraces full but not impossible. July and August get genuinely hot (35°C and above is common) and the city empties out as Bucharestians decamp to the Black Sea or the mountains; some restaurants close for a fortnight.
Winter is cold (often below zero in January) and can be snowy, but the bars and cafés are warm, the markets sell mulled wine, and the city is much cheaper than in high season. Spring comes late — April can still be raw — but by early May the chestnuts on Calea Victoriei are in flower.
The best single event to time a visit around is the George Enescu Festival, held in September every odd-numbered year (next: 2027), which fills the Athenaeum, Sala Palatului, and other venues with major international orchestras. Other dates worth knowing: the TIFF Bucharest indie film festival in May, the Diploma art and design show in September, and the Christmas market in Piața Constituției in December.