The summer season on the Black Sea coast runs roughly from mid-June to mid-September — warm sea (22–25°C), reliable sunshine, busy beaches, prices doubled at Mamaia. The peak is the first three weeks of August, when the Romanian summer holidays concentrate the crowds; if you want the beach but not the crowds, aim for June or September.
The old town is at its best in late spring (May–early June) and early autumn (September–October) — warm enough to sit outside, far quieter, and the Casino and Aquarium are open without the long queues. Winters are cold, windy and grey; the city is by no means closed, but you will mostly have it to yourself. The Limanul Mamaia music festival in late August and the George Enescu Festival events that occasionally come to the Casino are worth timing for.