Samos has a long summer season — May through October — and a wet, mild winter when much of the tourism infrastructure shuts down. June and September are the best months: warm enough to swim, far less crowded than July and August, and the wildflowers (June) or grape harvest (September) are in full swing.
July and August are when the island is busiest. Italian and German charter flights pile in, hotel prices roughly double, and the popular beaches at Tsamadou and Psili Ammos fill up. The summer meltemi wind can also kick up sea on the north coast in late July and August, sometimes making swimming uncomfortable for a few days. The wine harvest in mid-September is a good time to visit Pythagoreio's August festival of Aug 6 (Transfiguration) and the Heraion archaeological site on cooler days.