Stormzy might be headlining later but Glastonbury is waking up to blazing heat on Friday, with temperatures expected to reach 28 Celsius.
The festival is giving away free sun lotion and water to combat the heat, and extra shaded areas will be opened if necessary.
175,000 people are expected to attend the festival over the next three days.
Abba tribute act Bjorn Again will open the main stage, with Lauryn Hill and George Ezra playing before Stormzy.
But the music started early on Thursday night with a surprise set by Jamie Cullum at the BBC Introducing Stage.
The jazz pianist covered The Beatles' Come Together and Taylor Swift's Shake It Off, encouraging fans to post the latter to YouTube in the hope that she might ask to meet him.
Glastonbury's co-founder Michael Eavis also popped up at the Avalon Café, playing a short set with his band Michael Eavis and Friends.
Introduced on stage as the "Godfather of festival fun" and "chief cow herder", the 83-year-old warmed up the crowd with Elvis standards like Can't Help Falling In Love and Suspicious Minds.
He later brought out his daughter, Emily, to duet on Frank and Nancy Sinatra's Something Stupid.
Over at the West Holts Stage, meanwhile, thousands of people gathered to watch England beat Norway and reach the semi-finals of the Women's World Cup.
Among the audience was singer Billy Bragg and former England player Emily Westwood.
According to overnight figures, an average audience of six million people watched the Lionesses' 3-0 win, with the BBC One broadcast peaking at 7.6m in the closing stages.
The game was also watched by a party of three brides - one of whom got married at the festival earlier in the day.
"This is my spiritual home," Kerry told BBC 5 live. "There are people I only ever see at Glastonbury who are very close to my heart - so what better place to do it."
Up at the post-modern party area, Shangri-La, politics mixes with the music. This year's theme is "Junxtaposition", with huge art installations carved out of recycled rubbish to represent a post-Brexit wasteland.
The festival is making efforts to become more sustainable, banning plastic water bottles and cutlery around the 900-acre site.
Thousands of people also joined a climate change protest on Thursday afternoon, eventually forming a human sculpture in the shape of an hourglass to symbolise extinction.
The festival starts in earnest on Friday morning, with the full weekend line-up including Tame Impala, The Killers, Billie Eilish, and Janet Jackson, with pop star Kylie Minogue playing the Sunday afternoon "legend slot".
"It's a big deal for me. I keep calling it the afternoon slot just to take the pressure off," she told the BBC.
Minogue was due to headline the festival in 2005, but had to pull out after being diagnosed with breast cancer.
"All the more reason to celebrate," she said. "I made it. I made it here".
There'll be full coverage of Glastonbury on BBC TV, Radio, online and the BBC Sounds App throughout the weekend.