Factbox-A few facts about Glastonbury

LONDON (Reuters) - Glastonbury is the world's biggest open air arts and music festival.

It covers 900 acres in the mystical Vale of Avalon, where legend claims King Arthur was buried, Joseph of Arimathea walked and where leylines converge.

The festival is notorious for its torrential rain after three 'washout' years in 1997, 1998 and 2005 in which the entire festival site on Michael Eavis's Worthy Farm, deep in Somerset, became a slippery quagmire. Some fans revel in the mud.

Among the headline acts at this year's festival is U.S. rapper Jay-Z, Crowded House, Mark Ronson and James Blunt.

The three-day event also has poetry, theatre, circus and comedy.

In previous years performers have included such famous names as Paul McCartney, Rod Stewart, David Bowie, Tom Jones and Robbie Williams.

The first festival was held in September 1970 over a two day period with acts including Marc Bolan, Keith Christmas, Stackridge and Al Stewart. About 1,500 attended with admission being just one pound - but that included free milk from the farm.

In 1971 the festival moved to the time of the Summer Solstice and was known as the "Glastonbury Fayre". It was paid for by supporters of the ideal so the entrance was free and took a medieval tradition of music, dance, poetry, theatre, lights and spontaneous entertainment.

The first "pyramid" stage was constructed out of scaffolding and expanded metal covered with plastic sheeting, built on ancient Glastonbury-Stonehenge ley line. Acts included Hawkwind, Traffic, Melanie, David Bowie, Joan Baez and Fairport Convention watched by an estimated 12,000.