The early birds will all be vying for the prime camping spots next to the main stages where Foo Fighters, Radiohead, Stormzy and Lorde will perform over the weekend.
They'll be joined by 177,000 other revellers, musicians, stall holders and volunteers, who make the annual pilgrimage to Worthy Farm to drink in the proceedings before proceeding to the drink.
And while the only thing you can predict about Glastonbury is its unpredictability, here are 12 things we're 99% certain will happen over the next five days.
Yes, the UK is in the middle of a heat wave. Yes, the forecast says it will be dry. But if you turn up to Glastonbury with nothing but a pair of flip-flops and a floaty dress, you only have yourself to blame.
Playing the Pyramid Stage is a huge honour and a terrifying prospect, all rolled into one globally-televised gift box.
We've seen experienced bands like U2 and Gorillaz come unstuck and walk away humbled. But we've also seen underdogs like Pulp and Florence + The Machine triumph after being bumped into headline slots at the last minute.
Who's going to get it right - and wrong - this year?
Like these ones.
Every year, festival founder Michael Eavis holds a press conference. And every year he claims the latest Glastonbury festival is better than every other Glastonbury festival ever, full stop, no comebacks.
He will also drop a hint about next year's headliners ("they have six legs and they're from Cardiff"), prompting at least seven speculative articles on the NME website.
In 2014, we stumbled across a Korean funk band called Sultans of Disco playing in The Rabbit Hole at two in the morning.
Wearing matching dogtooth suits and playing an exuberant cover of the Ghostbusters theme, they were literally the best thing we saw all weekend. And yes, we were sober.
That's the magic of Glastonbury: at any given moment your new favourite band could be around the next corner. (It could equally be a doughnut van - there seem to be a lot of those too.)
If we had to single out a few rising acts, we'd say Norwegian pop singer Sigrid, gravel-voiced soul singer Josh Barry and folk trio Wildwood Kin will be among the acts winning people's hearts this year.
Every year, thousands of mangled tents and sleeping bags are discarded by campers who can't be bothered to clean up after themselves.
It costs the organisers Â£780,000 to clean up the mess, money that would otherwise be going to charity. So have some respect and bring your rubbish home with you.
OK, so Galway Girl is essentially an insult to the people and music of Ireland. But Ed Sheeran just about makes up for it with his impressively charismatic and likeable stage shows.
With years of incessant gigging under his belt, he's adept at working a big crowd - armed with just a microphone and his teeny, child-sized guitar.
Eschewing a backing band, he builds his songs from scratch on stage, using a sampler and a foot pedal to build inventive layers of loops. The fact the technology could fail at any moment gives his gigs a sense of peril that more polished headline acts lack.
Expect him to convert a few sceptics when he closes the festival on Sunday night.
Where does it come from? How does it stay upright? What if someone gets trapped under it?
Bonus points if it's taller than the bus.
This always precedes a video of someone juggling. Which is the perfect time to put the kettle on and be thankful you're not at Glastonbury, being forced to watch someone juggling.
Our mate heard it from the security guy on the Avalon Stage, who saw the setlist and swears it's true.
Also, Rihanna is doing guest vocals on Get Lucky and we saw Bryan Ferry having a kebab in the Stone Circle.
Don't try to retrieve it, whatever you do.
2017 has been almost uniformly rubbish, but standing in a field with 100,000 strangers, drinking sambuca from a plastic cup while Katy Perry belts out Firework in a spangly leotard will leave you with an enormous sense of well-being. Parklife!
The BBC will broadcast more than 30 hours of Glastonbury on TV this weekend, with further coverage on Radio 1, Radio 2, Radio 3 and 6 Music, the BBC iPlayer and online. Find out more on the official BBC Glastonbury website.
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Glastonbury has opened its gates and fans have started to trickle in.