Glastonbury Festival
One day, a farmer decided to set up a music festival. Michael Eavis had seen a Led Zeppelin concert and was a bit influenced by the hippy movement, although not that much, as he's a practising Christian.
This was the result. Taking place most years (there are breaks every five years to allow the land to recover) at Worthy Farm, near Glastonbury, Somerset. In The West Country if you need a better generalization.
Glasto, as it's commonly known, has become famous for people frequently sneaking in without paying, the Pyramid Stage and frequently becoming a mudbath. Consequentially, it has become synonymous with wellie boots. There has been violence at the past and a lot of arrests for drug possession.
In the past 20 or so years, the festival has been dominated by Alternative Rock, and Britpop in particular. The 2008 Festival (27–29 June) was the first for 15 years not to sell out before it began, probably due to the controversial decision to have Jay-Z (a hip-hop act) headlining.
The American equivalent (in terms of importance, attendance and general structure) is the Coachella Valley Music and Arts Festival in California, but that only started in 1999.