Green Man Festival takes place in Crickhowell, Wales, United Kingdom, usually in August, British summer. It is a 25,000-capacity festival, running 4 days across mountain with camping on site.
The Brecon Beacons make their own weather: sun, hail and mud in one afternoon. Waterproofs are worn over the nice outfit, not instead of it.
Days sit around 19°C (66°F) and nights fall to 10°C (50°F), a 9-degree swing that most first-timers underestimate. Rain is likely at some point across the weekend. Combined with the ground here, that means mud, which is the single biggest factor in what you should put on your feet.
Below: the 10 outfits that work best at Green Man Festival, what the crowd actually wears, and the practical rules, bag size, footwear, night layers. That decide whether the outfit survives the weekend.