Bonnaroo takes place in Manchester, Tennessee, United States, usually in June, US summer. It is a 80,000-capacity festival, running 4 days across open field with camping on site.
Tennessee farm humidity plus four days of camping. The Bonnaroo veteran look is deliberately low-effort: cut-off shorts, a bandana and shoes you have already written off.
Plan around heat first: daytime highs sit near 31°C (88°F), which rules out anything heavy, tight or synthetic in the middle of the day. 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 Bonnaroo, what the crowd actually wears, and the practical rules, bag size, footwear, night layers. That decide whether the outfit survives the weekend.