May, June, and September are the best months. The weather is warm and sunny, the sea is swimmable, and the town has not yet (or has just shed) its summer crowds. Hotel prices are considerably lower than in peak season.
July and August are high season. Rovinj is extremely popular and the old town fills to capacity, with queues at restaurants and intense competition for parking. If you come in summer, book well ahead and be prepared for crowds. The upside is that the full range of boat trips, restaurants, and nightlife is operating.
October can still be pleasant — temperatures in the low 20s, the crowds gone, and Istrian truffle season at its height. The autumn light on the harbour is beautiful.
Winter is quiet, with some restaurants and shops closing. The town reverts to its local character, which has its own appeal. Temperatures are mild by northern European standards.
The Rovinj Summer Festival runs through July and August with open-air concerts and the outdoor art exhibition on Grisia Street. The Batana Festival in August celebrates the town's traditional fishing boats.