It doesn't get much cheerier than the scene round about midnight on Friday at the Firefly Music Festival. The world's greatest surviving Beatle was bringing his headlining set at the largest music fest on the east coast to a crescendo by leading scores of thousands of fans - most more than forty years younger than he - in a singalong, dance-in-the-mud version of "Hey Jude."

For Paul McCartney, who turned 73 on Thursday, the celebration had begun two hours earlier when he began his set on the main stage at The Woodlands at Dover International Speedway singing an exultant, rocked-out rendition of his former band's "Birthday" before a sold out crowd of 90,000 spread over Firefly's sprawling grounds, which had been soaked with heavy rains earlier in the week.

"Good evening, Firefly!" the enduring cute Beatle said in greeting the crowd early on, before following "Save Us" from his 2013 album New with a robust "Got To Get You Into My Life." "It's a bit of a party, isn't it?"

It was. Beach balls were flying throughout the day, during performances by bands like the Durham, North Carolina neo-trip-hop duo Sylvan Esso - one of over 40 acts playing each day on seven stages throughout the Woodlands' green, whimsically decorated grounds, where Kings of Leon were scheduled to headline on Saturday and The Killers to close out the sold out fest on Sunday.

And the mood was generally joyful throughout McCartney's 32 song set, which found the genial septuagenarian, who started off sounding a bit hoarse but loosened up as the night wore on and he bopped through Beatles crowd pleasers like "Ob-La-Di, Ob-La-Da" and "Back In the U.S.S.R." as well as 1970s Wings highlights such as ""Band On The Run" and "Live and Let Die," the latter's melodramatic chorus  punctuated by exploding fireworks.

But it wasn't all mirthful musical merriment for McCartney, who will stay in the area to perform at the Well Fargo Center in Philadelphia on Sunday night. Early on Friday, he made mention of the nine victims of a massacre in a historic black church in South Carolina in dedicating "The Long and Winding Road" to "the people of Charleston. While we're all here, we need to pray for peace and harmony." Later, he sang a lovely solo acoustic version of "Blackbird," which he reminded listeners was inspired by  the Civil Rights struggles of the 1960s.

There was a sense of seriousness of purpose, too, to the acts that came immediately before McCartney. To his left on the seriously muddy Pavilion stage, the  interracial rap duo of Killer Mike and El-P  known as Run The Jewels, one of Firefly few hip-hop headliners (Snoop Dogg is playing Sunday night) delivered a hard hitting, commanding set.

Pulling equally from their two self titled albums - the latter of which was one of the standout releases of 2014 - the politically minded tandem dedicated "A Christmas F------ Miracle" to the Charleston victims, as well as sending out "Early" to "anyone who was ever shot and killed unjustly by a ... police officer." They also talked about an upcoming  tour of South America, where they'll take with them "the ridiculous, tragic arrogance that America has bred into it" with them. that was followed by chants of "U-S-A!"

On the main stage McCartney was preceded by another famous British vegetarian: Morrissey, the mordant Mancunian former leader of The Smiths, was was scheduled to headline the Academy of Music on Saturday.   Opening with Suedehead from his 1988 solo album Viva Hate, he asked the sparse crowd the musical question, "Why do you come here when you know it makes it hard for me?" Backed by an occasionally lumbering band and plagued by some sound problems, he was effective nonetheless, whether empathizing with sufferers in "World Peace Is None Of Your Business" ("Oh Egypt, Ukraine / So many people in pain") or wishing for the end of everything in "Everyday Is Like Sunday." With his closing "Meat Is Murder," however, he drove people from the stage, in some cases to the cheese steak stand.

Earlier on, Brooklyn electronic band Big Data proved to be an amusing surprise, with a set that playfully satirized digital culture in the surveillance age. Before the group took the stage a disembodied Siri-like voice introduced the crowd to "your new favorite band " and announced that during the set, the band would be "collecting you personal data and selling it back to you."

And as the heat of a muggy, 90 degree day bore down on the 90,000 - more than two-thirds of whom were camping out for the duration, making driving in and out of the fest surprisingly easy, even on a Shore traffic Friday -  Philadelphia power-pop band Cheerleader took shelter on the shaded Coffeehouse stage. The group - one of three Philadelphia acts, along with hip-hop duo Chiddy Bang and electro-pop duo Marian Hill, who played a late night, post McCartney set on Friday - put on a strong showing with tune drawn from their inviting debut album The Sunshine of Your Youth, including the particularly fetching "A Million Ways."  Singer Joe Haller made sure to thank the Cheerleader converts who had also seen the band earlier on the Forest stage and decided just once on a sweltering Friday was not enough.

Besides the music, what makes Firefly, now in its fourth year, such a massive draw for the millennial masses with flowers in their hair and funny hats on their heads is its ready-to-party presentation on grounds that would appear surprisingly bucolic to motorists accustomed to being stuck in Route 1 traffic alongside the hulking Dover Downs.
There are plenty of diversions, from lounging in the Hammock Hangout to listening in on a headphone concert in the wooded glen The Thicket, or sampling eight beers on tap brewed by Milton, De craft beer purveyors Dogfish Head in a vast air conditioned tent, which will be converted into a dance hall for next weekend's Big Barrel country music festival.

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