July and August are the high-summer peak, with temperatures in the high 20s to low 30s and the resort full of European holidaymakers. The beach absorbs the crowds reasonably well; the clifftop promenade and the restaurants feel busier.
May, June, and September are the sweet spot — warm enough to swim (the Atlantic is around 21°C in September), much quieter, and noticeably cheaper. The Portimão Sardine Festival in early August is a good reason to time a visit if you happen to be in the area.
Winter is mild (15°C daytime), often sunny, and very quiet. Many of the seasonal restaurants close, but the cliff walks and the long empty beach have a stark beauty.