Green River sits at 4,100 feet in high desert. Summers are hot — 95 to 100 F is normal in July and August, and the rocks hold heat into the night. Winters are cold and dry, often below freezing at night, but with sunshine and snow rare enough to be photogenic when it falls.
The best months are April–May and September–October. The river is high in spring with snowmelt, perfect for rafting Labyrinth Canyon; autumn brings cooler air, the melon festival, and crowds thin out across the surrounding state and national parks.
Melon Days in mid-September is the town's one major festival — a parade, a melon-eating contest, and free melon for all comers. It started in 1906 and is the best weekend to plan around.