Late April through May is one of the best windows: the vines are coming into leaf, the valley is intensely green, and the weather is mild without being hot. You'll have the roads and the wineries largely to yourself. September and October are the other peak season, and arguably the more dramatic one — harvest brings activity to every hillside, the light is golden, and the air smells of fermenting grape juice. Most wineries are willing to show visitors around during harvest, and the restaurants cook with a particular energy when the new wine is arriving.
Summer (July and August) can be hot, especially in the valley bottom where temperatures occasionally push past 35°C. The slate hillsides store heat and radiate it back at night. It's not unpleasant if you're used to it, but plan outdoor activities for mornings and evenings. Winter is cold and wet — the valley catches Atlantic weather coming in from the northwest, and January and February are genuinely grey and damp. The landscape has its own beauty in winter, and you'll find the wineries' tasting rooms quiet and welcoming, but it's not the time to come for walking or vineyard visits.