Dance fever at Boston Calling, capped by a Mumford sing-along

Mumford & Sons played Boston Calling’s main stage Saturday night.Ben Stas for The Boston Globe/Globe Freelance

And there it is. With a long, mood-shifting set on Boston Calling’s main stage Saturday night, Mumford & Sons cued itself up for a viral moment and then delivered. On a day that included mixed messages about the vitality of rock music in today’s market, the group sent much of the audience away humming a Beatles tune.

The song in question was “With a Little Help From My Friends,” and Mumford & Sons closed the day by crowding the stage with guests to best get the point across. These included Aaron Dessner of The National, a curator of Boston Calling, and Brandi Carlile, who played her own well-received set earlier in the day before enthusiastically belting out lines from the verses of the classic-rock sing-along.

It wasn’t a new move for Mumford, which has also closed nights at the Glastonbury and Bonnaroo festivals with guest-laden versions of the Lennon-McCartney chestnut. But it was a bases-clearing gesture, pitched at full intensity.

A Dessner-aided romp through “Hopeless Wanderer” also succeeded mightily, following faithful renditions of Peak Mumford tunes like “The Cave” and “I Will Wait” which packed more punch than the indistinct rock sounds of the group’s latest material — proving that a few musicians ripping on their acoustics in the manner of Americana can be the most dance-inducing move, even for a festival crowd.

Earlier, Tegan and Sara delivered a feel-good set of dance-pop. Complete with a stage set including inflatable letters declaring the Quin sisters’ initials, and an LED display that favored pastel-colored designs, this was electro-twee to the extreme.

Backed by a trio, each of the ladies had featured moments and occasionally worked some guitar or synth work into the mix, though their act was at full charm level when they both stalked center stage with just microphones in hand. “Boyfriend” scored big, as did “Back In Your Head.” Tegan and Sara have always had pop instincts, but while it took them years to wind fully into this day-glo world of glossy synths and electronic beats, the setting suits them well.

The xx marinated in a stylish, spare groove that, for all of Oliver Sim and Romy Madley Croft’s enthusiastic side-to-side swaying, didn’t connect very deep into the concert grounds. What might have been hypnotic under different circumstances seemed to evaporate into listlessness under the afternoon sun.

Perhaps the set gained momentum, but I headed over to catch the 1975 and saw one of the more vital sets of my time at the festival. Like much of Saturday’s music, this band’s sound was heavily retrofitted with 1980s synth textures and seemed to deliberately avoid the sweaty immediacy of rock music or its harder-edged siblings. Matt Healy wore a blousy, white, long-sleeved shirt that visually complemented the Prince-like rhythm guitars (if not the blocky, Peter Gabriel beats) in “Love Me” and the irresistibly winning “Girls.”

“She’s American” was slick, forceful, and dance-mandatory in a way only hinted at on the studio cut. Moodier numbers like “Somebody Else” also had more ballast in the live setting. The 1975 is a compelling live act, and at times the band was fully swinging, though the pastiche-like nature of its sound sometimes interfered with the momentum — as if there were too many Reagan-era pop moves to borrow at once, with no space left to dig into a groove and just stay there.

With Wolf Parade, Weezer, and Tool set for prime festival slots on Boston Calling’s third and final day, the rockers were poised to have their say . . . though the youngest band in that trio debuted 14 years ago. The second day of Boston Calling was more about full-band dance sounds — with a late-’60s sing-along to send everybody home.

BOSTON CALLING

At Harvard Athletic Complex, Allston, Saturday