Monteverde sits in the clouds for most of the year — that is the point. Temperatures stay in the low 20s °C by day and drop to single figures at night, and you should expect rain or mist at any time. December to April is the official dry season; visibility is best then, though the wind can be ferocious on exposed ridges.
May to November is the green season — more rain, fewer crowds, lower prices. Birds are easier to spot during the resplendent quetzal's breeding season from March to May, when the males display in the upper canopy of the cloud forest reserves. October is the wettest month and several smaller operators close briefly for maintenance. Whenever you come, bring a waterproof jacket, a fleece, and quick-drying clothes.