Fans of the Firefly Music Festival waited 837 days for the return of the grand gathering at The Woodlands of Dover, Delaware, and then had to wait a little bit longer for Billie Eilish.

Torrential rain and lightning strikes Thursday along Delaware Bay — where the festival’s bucolic 105-acre site is located across Route 1 from the hulking Dover International Speedway — meant the start time had to be pushed back 4 ½ hours to 7:30 p.m.

Some bands scheduled early in the day had their performances nixed and are hoping to play later in the weekend at the camping fest, which is the largest of its kind on the East Coast and expects to draw 50,000 concertgoers each day through Sunday.

Others performers were shuffled into later slots on Thursday, which made for a madcap — and muddy — Thursday evening for festival goers who endured long lines at health screening trailers to check for proof of a vaccine or negative COVID-19 test required for entry before trudging or taking a shuttle across the highway.

A busy night of music was spread across five stages, some nestled in the woods like the cozy Treehouse, where Del Water Gap (the project of songwriter Holden Jaffe, who’s from Connecticut) and Nigerian American singer-rapper Serena Isioma played.

The grounds were lit up with sparkler towers that looked like perpetually igniting fireworks — as well as by festivalgoers who encased themselves in lights (but rarely wore masks) as they made their way around grounds that were not-too-slushy but treacherous in spots. (On the bright side, it barely rained, once the music started.)

The stylistic range of act was wide — from buzzy Norwegian indie songwriter Marie Ulven Ringheim (who performs as Girl in Red) to Austin, Texas, electronic duo Missio to Baltimore hard-core punk band Turnstile.

But really, Thursday night at Firefly was all about Eilish. The still teenaged star — she turns 20 in December — who dominated the 2020 Grammy Awards with her goth-pop breakout debut When We All Fall Asleep, Where Do We Go? is part of a festival that usually takes place in June and has taken care to book big name headliners after being canceled in 2020 due to the coronavirus pandemic.

Eilish is playing a limited number of festival dates in support of her chart-topping new Happier Than Ever album before embarking on a 2022 tour that will bring her to the Wells Fargo Center in South Philly on Feb. 13.

Friday night’s scheduled Firefly headliner is The Killers, the Brandon Flowers-fronted Las Vegas rock band who played a sold-out warm-up gig at the Franklin Music Hall in Philadelphia on Wednesday, plus Pittsburgh rapper Wiz Khalifa.

Saturday’s bill is topped by Australian psychedelic rockers Tame Impala, with formerly Philadelphian deejay Diplo spinning late into the night. The festival closes Sunday with rapper-singer-flautist Lizzo, with Megan Thee Stallion and Philly indie acts Mt. Joy and Orion Sun also scheduled.

Day passes for the festival are available at fireflyfestival.com. The best source for updated schedule information is the Firefly Festival mobile app. Sets by many of the festival’s biggest names can be viewed for free throughout the weekend on the Mandolin live streaming service at firefly.mandolin.com.

On Thursday, the Eilish anticipation was keen. A highlight of Tate McCrae’s set, which immediately preceded the headliner, was “Tear Myself Apart,” a song that Eilish and her brother, Finneas O’Connell, wrote for the Canadian teen pop star.

Phoebe Bridgers played a sterling set on the fest’s main Firefly stage, pulling from her stunning 2020 album Punisher and 2017′s melancholy Stranger in the Alps, with J.J. Kirkpatrick’s trumpet standing out on a cover of Bo Burnham’s “That Funny Feeling.” Midway through the set, Bridgers copped to being an Eilish superfan, saying that her Firefly booking was “an elaborate plot to see Billie.”

Opening with “Bury A Friend,” the song with a lyric that supplies Where Do We Go’s title, Eilish was greeted with screams of delight from the crowd that had converged from the fest’s various stages.

Backed by only two musicians — her multi-instrumentalist brother and drummer Andrew Marshall, both of whom also supplied live production effects — Eilish put on an 80-minute show that was a minimalist marvel.

From the visual presentation — with the stage sometimes bathed in red, other times with the silhouette of a giant spider looming behind her — to the disciplined musical approach, Eilish is a savvy performer who never overplays her hand.

Her emotionally acute songs don’t sugarcoat adolescent experience and are often built on whispered vocals and pared down arrangements — a prominent bass line here, a simple repeated keyboard figure there — that are rhythmically adept and pop savvy.

After such a long layoff, Eilish’s Firefly show was a necessary celebration for everybody involved. “Guys, I can’t tell you how many times I dreamed this,” she said of being back on stage, while admitting to nervousness, in one spoken interlude. (In another, before “all the good girls go to hell,” she talked trash to climate change deniers.)

Fans shouted along to self-assertive songs like “Therefore I Am” and winning put-downs of woeful lovers such as “Lost Cause,” and were delighted by an unscripted moment when she jumped off the stage and granted the request of a fan to sign his chest.

But the most impressive part of the show wasn’t the pandemonium. It was the quiet, the way Eilish was able to command a vast ready-to-party space with understatement.

The crowd couldn’t quite bring itself to put their phones down and be entirely in the moment for “Ilomilo,” but they were right with her when she put body-shamers in their place in the spoken interlude “Not My Responsibility.”

And it was a wonder to behold a festival audience remain rapt when Eilish and her bandmates really brought the tempo down and displayed a facility with pre-rock pop with a tendency toward torch song on “Billie Bossa Nova” and “Halley’s Comet.”