Thousands flocked to Zilker Park Wednesday evening for an all-star cast that emphasized the “blues” of Blues on the Green, KGSR’s long-running free summer concert series. Summer might not officially arrive for another few weeks, but this felt like the cultural christening of the season.

It was certainly hot enough. Though temperatures weren’t off the charts, the humidity was, with stiflingly sweaty and overcast conditions that seemed to threaten rain but held off and allowed the show to go on.

Mike Flanigin and Charlie Sexton high-five as Billy Gibbons and Jimmie Vaughan step out front at Blues on the Green in Zilker Park on May 25, 2016. Photo by Erika Rich for American-Statesman

The evening centered around B3 organ master Mike Flanigin and his all-star cast of guitar greats. ZZ Top’s Billy Gibbons kicked things off with “The Drifter,” the title track to Flanigin’s 2016 album that gathered a bunch of blues, rock, jazz and soul ringers from Austin and beyond. Flanigin showcased that album last August at the Paramount; this time it was more just a launching point for deeper blues explorations in the hands of Gibbons, Jimmie Vaughan and Charlie Sexton, with George Rains keeping the beat steady on drums.

PHOTOS: Jimmie Vaughan, Billy Gibbons perform at Blues on the Green

All three guitarists took lead vocals, as did Flanigin from his seat at the B3. Flanigin moved to bass at the end of the night for Gibbons’ signature “La Grange,” which they followed with an unscheduled encore after their main set finished a tad before the 10 p.m. curfew.

If the heat was taking its toll on the crowd, who flocked to water stations and an Amy’s Ice Cream booth for relief, they didn’t seem to mind. Indeed, it seemed to fit the sweltering swagger of the music, recalling a hot July night at Austin blues haven Antone’s. Fittingly enough, just as the concert was starting, the recently reopened Antone’s announced the lineup for its two-week 41st anniversary celebration in early July. Shows include Chicago blues veterans Henry Gray (July 1) and Eddy Clearwater (July 2), an 87th birthday party for hometown heroine Miss Lavelle White, a reunion of singers Angela Strehli, Lou Ann Barton and Marcia Ball (July 13) and the Dirty Dozen Brass Band (July 16).

RELATED: Mike Flanigin’s “The Drifter,” Austin’s best album of 2016

The Peterson Brothers at Blues on the Green in Zilker Park on May 25, 2016. Photo by Erika Rich for American-Statesman

Also in the anniversary lineup are the Peterson Brothers, who kicked off Wednesday’s Blues on the Green concert with a tight and joyous set that energized the crowd. Led by Bastrop teens Glenn (guitar) and Alex (bass) Peterson with Chris Mead on drums, they favored feverish blues tunes with punchy solos, but took it down a notch midset with a beautiful version of “Amazing Grace” that featured Alex on violin.

Next up for Blues on the Green is Wild Child with Max Frost on June 22, followed by Hayes Carll with Carson McHone on July 13 and finally Sweet Spirit with the Suffers on Aug. 3.