The Leeds band plays an unadvertised set on The Other Stage at 11:00 BST, 12 hours before their main show on the John Peel Stage.
Blondie, Lily Allen, Rudimental and Paolo Nutini are also on Friday’s line-up.
Canadian band Arcade Fire tops the bill, playing the main Pyramid Stage at the end of the day.
Revelers have been gathering on Somerset’s Worthy Farm since Tuesday. About 85,000 had arrived by Thursday evening, with 75,000 more due by Friday night.
Tragedy hit the site on Wednesday when a 67-year-old woman died in her sleep.
Police said the festival-goer died in her tent overnight, from what they believed to be natural causes.
The 900-acre site saw its first drop of rain on Thursday afternoon. By early evening, the paths and fields were muddy underfoot, but worse is to come.
The Met Office has issued a yellow weather warning for Saturday, with thunder and lightning predicted in the south-west of England.
However, festival organizer Michael Eavis was more optimistic.
The 78-year-old, who established the festival on his dairy farm in 1970, is now a celebrity in his own right – constantly asked to shake hands and pose for photographs as he inspects the site.
Arcade Fire take to the stage at 22:00 BST.
Ahead of the show, guitarist Richard Reed-Parry said the band “may have Somerset artists” on stage with them.
The annals of rock history do not list Somerset as a source of tremendous inspiration – leaving Arcade Fire to choose from the likes of Kula Shaker, Reef and, perhaps most excitingly of all, The Wurzels.
But Glastonbury is, as ever, a hotbed of rumors. Whispers are circulating that David Bowie and Jonathan Ross, who both appear on Arcade Fire’s new album, will take to the stage on Friday.
Fans are also speculating that Coldplay, The Libertines and Radiohead could make impromptu appearances.
And in the Tipi field, one reveler swore blind that Metallica’s place at the top of the bill on Saturday was an impressive bluff – with funk-rock superstar Prince destined to take their place.
Back in the real world, Mumford and Sons – who closed the festival last year – have announced a special “mini-festival” in the Avalon Cafe, situated in the remote Avalon field, on Saturday.
Other acts confirmed for the rest of the weekend include Dolly Parton, Lana Del Rey, Ed Sheeran and Ellie Goulding.
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