Philip Glass, in blue, performing with his ensemble.

Yesterday was hot — miserably hot. We were loath to go outside. We lingered in the doorways of air-conditioned restaurants and corner delis (we even bought things we didn't want from those delis just to bask in a brief respite of cool). The number of passersby on the street The Observer heard mutter, "It's so fucking hot"? Nine.

The number of music aficionados, free-outdoor-concert enthusiasts, and liberal artsy types of all ages willing to brave the heat for a concert by Philip Glass in Rockefeller Park on Wednesday night? As many as 5,000.

Part of the annual River To River Festival, the concert was the classical music giant's only free gig of the year. The hodres kept streaming in — picnic blankets, Whole Foods takeout, and lawn chairs in tow — and the excitement was tangible. As a fan told The Observer, "This is big. When Glass gives a free show. You go. Period."

But before Glass took center stage, the concert opened with a performance by Face The Music, the city's only teen classical musical ensemble (the oldest member was born in 1994!), who played "Glassworks" — vintage Glass, "arguably the most classic Glass out there," The Observer overheard. To our right, a bespectacled, bearded hardcore Glass fan told us, "I saw 'Glassworks' back in '82. At Hahhhvard," he drawled. "Snuck into the auditorium and sat right down. No one saw! Haven't been able to get enough since."

At first, the crowd seemed less than enthusiastic about the adolescent ensemble. We overheard a fair amount of "When is Glass actually going to come on?"

But they spoke too soon. This was hardly child's play. The strings began. Then a piano. The crowd perked its ears, raised its eyebrows, and whispered a collective "wow." A moppy-haired boy — 11, tops, who bore a striking resemblance to the master himself — took to the electric keyboard, tearing it up with the quintessential piano-gone-wild cacophonic genius that is Glass. "Wow. These kids are amazing!" The crowd picked up their blankets and canvas CSA bags, and moved closer to the kids.

And sitting cross-legged in the grass among the laypeople was the master himself. Taking it all in. Alone and inconspicuous. No one seemed to recognize him, until a picnicker pointed a finger and raised an iPhone. A few cameras followed. But, all in all, the crowd was pretty blasé, taking another sip of their organic lemonade and keeping their cool.

Half an hour later, Glass went behind the main stage. The mini-Glasses stopped and it was time to face the master.

A fan sitting to our left, a white-haired woman who looked like she probably had a good time at Woodstock, told The Observer, "I first saw him in 1969." She took a swig from a bottle —"grapefruit shower gel" from the Body Shop, according to the label —smacked her lips and let a subtle smirk. Gin. "'69. Back in Poughkeepsie. It was like, 'oh my god.'"

Our small talk was cut short when WNYC radio personality John Schaefer got on stage. "This has been a big year for Philip Glass." On January 31st, Glass celebrated his 75th birthday. Other notables born on the same day? Franz Schubert and Justin Timberlake. "And few people would be happier than Philip Glass to share a birthday with both Schubert and Timberlake," Mr. Schaefer said.

Then it was Glass time. He was playing with his ensemble (aptly named The Philip Glass Ensemble), comprised of three pianists, one killer female pianist/vocalist, three on the horns, and an electronic sound maestro in the back. They opened with, "Civil Wars," a composition from 1982 and followed with "Music and Similar Motion," from 1964. Yes. 1964.

But our bootlegging neighbor was not satisfied. "This is so 20th century. Come on already. Show us something new." Another sip and a wink. "I expect a lot from my geniuses."

Everyone else, however, looked like they were enjoying themselves. 20-something couples with funny glasses and funnier hats lied down and drummed their fingers to the curious rhythms of Glass's piano. Two seventy-something men meditated. Little boys with long hair and cool-looking parents ran around the grass. The Observer smelled some wafts of weed (a quiet protest to the fight in Albany?).

Then it was screechcrackzzzzz. Ears plugged. But was it just part of the music? "Now, finally, that's something new!" said our increasingly tipsy friend. Another deafening screech. Nope. Philip Glass's piano had just broken.

He took the mic. "As you can see, we have a little problem that we need to deal with. Be back in a few minutes. It'll be a little cooler then."

Someone from the audience shouted, "Sing for us!"

Five minutes later, Mr. Glass and his crew were back. The sun was setting over the Hudson. Joggers and roller-bladers along the water had stopped and made their way over to the music. People in business suits toting briefcases down West Street stood on benches behind the stage. Some loosened their ties. The ever-increasing crowd looked more and more mesmerized as the longest day of the year was coming to a close. And it just wasn't the weed, the hidden gin in cosmetic bottles and flasks, the wine poured from paper sacks into Nalgene canteens.

Hands moving like mad, hunched over his piano, sitting up straight every couple of minutes and throwing his head back, Philip Glass was a force to be reckoned with.

Was it as good as Cambridge in '82, The Observer asked our bearded friend when the concert ended. "It's Glass. He's always this good."