Roscoff has a mild maritime climate, warmer than you'd expect for northern Brittany thanks to the Gulf Stream. That said, it rains and the wind comes in off the sea — this is the Atlantic coast, not the Mediterranean.
The best time to visit is May to June or September. The Jardin Exotique is in full bloom in late spring, the Île de Batz is uncrowded, and the weather is often crisp and clear. July and August are peak season — the ferry terminal is busy, prices are higher, and the town fills with English families who just stepped off the boat.
The onion harvest — the famous Roscoff pink onions (oignons roses de Roscoff) — runs from August into October. Local markets sell them in quantity.