Romania has four distinct seasons, and the right time to visit depends on what you're planning to do.
Spring (April–June) is the best all-round season. The countryside turns green, wildflowers blanket the meadows, and temperatures are pleasant across most of the country — warm in the lowlands, cool in the mountains. Crowds are modest and prices reasonable. Easter, celebrated with particular intensity in Romania (Orthodox Easter falls in April or May), is worth experiencing if you can time it right.
Summer (July–August) brings heat to the plains and the Black Sea coast. The coast becomes crowded and expensive in July and August, and even mountain resorts fill up. That said, summer is festival season — Untold and Electric Castle draw huge crowds to Cluj, and village festivals animate the countryside. Hiking in the Carpathians is excellent in summer, with long daylight hours.
Autumn (September–October) is arguably the most beautiful time to visit Transylvania and the mountain regions. Forests turn red and gold, the harvest is in, and tourist numbers drop sharply after mid-September. Temperatures are still comfortable. Wine country around Cotnari and Dealul Mare is at its best.
Winter (November–March) is cold and often snowy, especially in Transylvania and the mountains. The ski resorts at Poiana Brasov, Sinaia, and Predeal come into their own. Christmas markets in Sibiu, Brașov, and Bucharest are atmospheric and increasingly popular. Expect some rural roads to be difficult or impassable without winter tyres. The Painted Monasteries of Bucovina look particularly striking under snow, and you'll have many of them almost to yourself.
The Black Sea coast is essentially a summer destination — most hotels and restaurants close between October and May.