Any time Paul McCartney is booked to headline a festival, it’s cause for celebration. For attending fans, it's a chance to see one helluva live rock production and the closest thing most of us will ever get to a Beatles show. And for the other bands -- especially those slotted earlier on the same stage -- it's the benefit of a sizable crowd camping out all day for the most prime spot to watch Macca.

Lollapalooza’s organizers undoubtedly had those inevitabilities in mind when they devised one of the smartest possible schedules to kick off the first day (July 31) of the festival’s 19th year (this marked the 11th consecutive edition at Chicago’s Grant Park). What culminated was a monumental meeting of one living legend (McCartney) and two artists (Alabama Shakes and Gary Clark Jr.) who are likely to achieve that status all within the span of four short hours.

Even before that all-star run began, earlier acts performing on the park’s southern end reaped the benefits of those faithfully waiting for the grand finale. A decent-sized crowd had already gathered when up-and-comer SZA -- the 24-year-old, New Jersey-based singer born Solana Rowe, who signed to Kendrick Lamar imprint Top Dawg Entertainment in 2013 -- hit the main stage around 12:45PM for a relaxing R&B precursor to a day heavier on straight-ahead rock.

With an arsenal of dance moves to match the range of her voice (from sultry to peppy), plus a vibrant smile that complemented her bright, orange mane, SZA charmed the early-bird audience. Next up, British indie quartet Glass Animals drew enough of their own dedicated followers -- who showed their love by shouting along to the bulk of songs pulled from the band's 2014 debut Zaba -- to at least double those already planted for McCartney. Their music was more textured and nuanced than most young bucks recently gaining traction, and that’s a good thing -- they have the pop appeal needed for mainstream support and the creative spirit to keep it interesting all the while.

More of their contemporary ilk dominated Grant Park’s northern end in the meantime: England’s Coasts got things started, followed by a wildly energetic trip to musical church from St. Paul and the Broken Bones. It was after that Birmingham, Ala.-based outfit that Lollapaloozans got a taste of a man who may never reach the level of “legendary” but has certainly established himself as an icon of sorts already. That’s Father John Misty, the once-reserved yet wonderful backing vocalist and drummer for Fleet Foxes then known as J. Tillman, who's now an established solo artist with equal parts bohemian and sex appeal.

Throwing himself repeatedly to his knees while swinging his mic stand over and around his lanky frame, he and his band wowed with the opening thrust of “I Love You, Honeybear,” the title track of his recently released sophomore disc under the FJM name. During the following set of sardonic yet sweetly sung ballads -- including favorites like “Only Son of the Ladiesman,” “Bored in the USA” and “Hollywood Forever Cemetery Sings” -- he proved he’s simultaneously one of the best vocalists and most facetious characters currently on the scene. “I’m depressed,” he proclaimed at one point, to which one fan loudly responded, “No. You’re not!” Knowing laughter rippled outward through the audience.

At that point, it became a choice of “What do we want to dance to?” with Hot Chip and MS MR as competitors. We ultimately went with the latter New York pop-rock duo comprised of fiery-haired frontwoman Lizzy Plapinger and keyboardist Max Hershenow, who sparked a respectable flash mob with a good helping from their just-released second album How Does It Feel?

Their draw might’ve been larger had it not been for one of the most anticipated sets of the weekend, the Lolla debut of Alabama Shakes. Though Brittany Howard and Co. emerged with the immediately commanding “Future People,” they quickly reverted to a vibe that wasn’t exactly a fire-starter for a throng larger than any other all day. “Hang Loose” was a catchy but mellow groove, and “Miss You” followed by “Heartbreaker” were slightly too waltz-y for a crowd subjected for long hours to a relatively relentless sun. No coincidence then that the aptly titled “Rise to the Sun” marked the real party-starter for the group.

A focus on driving compositions laced with incredibly apt shredding from Howard on that track and others from April’s Sound & Color, like “The Greatest” and “Dunes,” showed the Shakes pulling ahead of the rest of the roots-rock-revival pack. It became more a sign of their might when the PA cut out for a few minutes toward the end of the latter tune. Very few seemed jolted enough by that to turn tail before they’d wrapped up on the powerhouse anthem “Gimme All Your Love.”

Besides, they only had to spin around and walk a few feet afterward to catch a rambunctious set from Austin-bred guitar maven Gary Clark Jr., who at just 31 years old is already stirring up talk about taking a seat at the table among the all-time gods of the axe. He clearly had a sense of the impact he could have sandwiched between the Shakes and Macca, and launched his set with shred-packed versions of first album cuts, “Bright Lights” and “Ain’t Messin’ Around.” He then showcased his inimitable versatility by previewing tracks from upcoming album, The Story of Sonny Boy Slim: “We’re Gonna Make It” mixed soul, hip-hop and ripping Texas blues riffs, “Our Love” reminded of his still flawless falsetto before it erupted into a solo punctuated by impressive hammer-ons and “Grinder” served as the swampy closing jam.

His set featured the sort of blistering fretwork that made a guest spot on McCartney’s closing run seem more than likely -- Clark’s licks would make an excellent addition the guitar-filled finale of “The End.” But no dice. And who can blame McCartney for omitting guests? His 31-song revue was so heavy on the best of the Beatles (“Got to Get You Into My Life,” “Let It Be” and “Hey Jude” inspired the most galvanizing sing-alongs), Wings (“Let Me Roll It” is still one of the most swaggering rock cuts ever penned and how ‘bout them pyrotechnics on “Live and let Die?”) and his various solo efforts (no Kanye cameo on “FourFiveSeconds,” but it was an awesome tease-intro for “We Can Work it Out”).

Yet apparently, there was one artist McCartney admired enough to bring out for a short guest spotlight: Brittany Howard. Wearing the smile of a multi-billion-dollar lottery winner, she joined him on lead guitar and backup vocals for “Get Back” at the beginning of a seven-song encore. And to think, just a few years ago she was still delivering mail for the U.S. Postal Service. Even without that dazzling performance, she and her band had begun carving out their own special space in rock and roll history -- it was only fitting that Macca should be the first to officially offer up the torch.

Check out photo highlights from Friday in the gallery above and check back in tomorrow (Aug. 2) for day two's talent, including Metallica, Brand New, Tame Impala and more.