Porec is a summer city, with most of its restaurants and many of its hotels closed from November to March. The Adriatic stays swimmable from mid-June to mid-October.
July and August are peak: lively, hot (often above 30°C), and packed with families on a one-week beach holiday. The water is at its warmest. The downside is the crowds — the old town can be busy enough by midday that movement slows, and the larger resort hotels can feel like seaside camps.
June and September are the sweet spot for most travellers. The water is still warm enough to swim, the days are long, and the old town breathes again. The truffle season runs into late autumn — late September and October are excellent for an inland food-and-wine trip combined with one last sea swim.
The Porec Open Air festival, with theatre and concerts in the basilica complex and Marafor Square, runs through July and August.