The best months for Paros are May to June and September to October. The island is warm, the ferries are running, the beaches are open, and the crowds are manageable. In May and June the hillsides are still green and the wildflowers are out.
July and August are high season. The beaches fill, prices rise steeply, and accommodation needs to be booked months ahead. The meltemi wind blows hard from the north from mid-July through August — excellent for windsurfers at Golden Beach, less welcome if you want calm water for swimming. The wind drops at night, making evenings pleasant.
September is arguably the best month: warm sea, thinner crowds, and the first hint of autumn light. October is still swimmable in theory but can bring the first autumn storms.
November to April, Paros is very quiet. Most tourist facilities close, ferry services thin out, and the island returns to its agricultural rhythms. It is not uninteresting in winter — the villages feel genuine and the light is beautiful — but you need to be comfortable with limited services.