The Glastonbury Festival of Contemporary Performing Arts was the result. Taking place most years (there are breaks about every five years to allow the land to recover) at Eavis's Worthy Farm, near Glastonbury, Somerset. In The West Country if you need a better generalization.
Glasto, as it's commonly known, has emerged from the counterculture/s of the 70s and 80s to become a major and mainstream event in British cultural life, something reflected by, and maybe to some extent because of, the The BBC's increasingly heavy television coverage of the festival starting in 1997. The late 90s/Millennium also saw an upping in security after years when the festival had been famous for people frequently sneaking in without paying. A watershed moment came in 2003 when tickets sold out within 24 hours of going on sale for the first time. Almost every year since has seen similarly immediate sales.
In fact the only year that tickets haven't sold out immediately since was 2008 which many theorised was due to another major shift in Glasto culture introduced that year: a hip-hop headliner in the form of Jay-Z. While the festival has always been diverse and evolving in the music it showcased, it would be fair to say that it was, until the last ten years, very much led by 'guitar music' and, well, pretty white. Jay-Z was not an uncontroversial choice of headliner with Noel Gallagher of Music/Oasis famously sneering at the idea. Nonetheless, in the decade since the trend has continued towards more pop and hip-hop acts alongside the traditional Glasto fare. Headliners have since included Creator/Beyonce, Kylie Minogue and Kanye West.
These shifts in culture have often been spearheaded by Emily Eavis, Michael's daughter, who started getting involved with the festival after her mother's death from cancer in 1999 and who is now a co-organiser. Both she her father are keen to keep the festival fresh and inclusive and make sure it doesn't become the province only of older white patrons. Michael Eavis has said he didn't want festival attendance becoming "too middle aged and respectable".
The most iconic images of the festival are the central Pyramid stage where the biggest acts are hosted, especially when seen at sunset with the famous Glastonbury Tor and Abbey a few miles away lined up behind it. And mud. The weather is, in fairness, often very nice, but the wet years have been frequent and memorable. It is a brave - or naive - festival-goer who doesn't take wellies to Glastonbury.
Despite the move into respectability, the vibe of Glastonbury remains in some contrast to other big UK and worldwide music festivals for still having a (wellied) foot firmly in the camps of the New Age, edgy, hippie and artsy. Whereas festivals like Reading and Leeds are purely concert-focused, Glastonbury is interested in creating a theatrical, immersive experience and involves artists, set-designers and other gonzo creators. Areas, current and extinct, like Lost Vagueness, Shangri-La, Downtown NYC and Block9 are almost redolent of Disney Theme Parks in their immersive world-building and theatrical trickery - albeit with more adult/countercultural theming. Disneyland never had a giant, flame-throwing mechanical metal spider as a DJ booth, a J. G. Ballard-esque apocalyptic council estate or a recreation of seedy 80s gay clubland in New York.
A fairly recent addition to the festival has been the Sunday afternoon "Legend" slot where an older act (usually a solo artist) who wouldn't normally be seen as a festival type act plays a set. These have generally been well received and the artists have been surprised by the enthusiasm of the generally much younger crowd for their songs. Highlights have included Tom Jones in 2009, Barry Gibb in 2017 and Lionel Richie in 2015 who went down a storm unlike headliner Kanye West who had a very mixed reception.
Coachella is often seen as the closest US equivalent by dint of audience size and the kinds of acts booked, but it is really only partly comparable. For one thing, it is much younger, having only started in 1999. You might have a closer idea if you imagined a cross between Coachella and Burning Man.
'Fallow years' where no festival occurs happen every four or five years, in order for the land - which is a working dairy farm (or rather, at this point, several farms) - to recover. Sometimes these off-years are arranged to work around other major events; e.g. 2012 when the Olympics was held in London. When Glastonbury is on, everything from police time to marquee hire is in short supply for anywhere else. The last 'fallow year' was 2018.
The festival was back for 2019, and had planned to mark its 50th anniversary in 2020 - but then was cancelled due to the ongoing COVID-19 pandemic. The BBC kept the Glasto-time in its scheduled and played past performances throughout the weekend Glasto 2020 should have happened, presented by the usual BBC coverage crew from a festival-less Worthy farm where cows grazed peacefully around the skeletal structure of the Pyramid stage. Michael and Emily Eavis have promised to be back with a delayed 50th anniversary event as soon as possible. The 2021 event was also cancelled as the pandemic continued to pose a risk. Tickets purchased for the now cancelled events were rolled over to the potential 2022 event.