President Obama and first lady Michelle dance at the Commander-in-Chief’s Ball as Jennifer Hudson peforms “Let’s Stay Together.” (The Washington Post)

The enormity of the occasion met the enormity of the room.

Like, wowza. What a room!

Acres of concrete floor at the Walter E. Washington Convention Center to sashay across in gowns and heels, to clippity-clop around in tuxes and shiny dress shoes. All in search of that special “I-was-there” moment, an inauguration night when a city consumed by its quarrels stopped its sniping for a few, and just smiled.

In this age of austerity, President Obama trimmed the fat off the inauguration festivities, slimming down from the 10 balls he attended for his first inauguration to the two official galas he attended Monday night for his second inauguration. But it’s not like he slummed it, not with Alicia Keys on stage at the piano in that red backless dress.

“I just wanted it to be me and you,” Keys cooed to the crowd. And, you know, thousands seem to believe her.

“Obama’s on firrrrrre!” she sang.

Oh, yeah. Not slumming it at all.

Not with Jennifer Hudson serenading the president and the first lady as they slow-danced to “Let’s Stay Together.” And yes, America, Jennifer belts out that one better than Barack, at least better than he did last year when he gave it a go at the Apollo Theater in Harlem.

The president got big cheers, but the crowd at the Commander in Chief’s Ball really lost it when he confided that “I’ve got a date with me.” His date-for-life, Michelle Obama, strode out in an ankle-length ruby-colored chiffon dress by ­Jason Wu, the same fashion designer who became a megastar when the first lady wore his white, single-shoulder gown to the inauguration balls four years ago. “She inspires me every day,” the president told the crowd.

Midway through their solo dance turn, the first couple parted, and each spun for the crowd for a few moments with a service member in uniform. The first lady got a peck on the cheek from the serviceman when they were through.

The guests at these sorts of things always need a little nudging. The Voice over the speakers gave instructions, as simple as “A. B. C”: “We don’t want you standing around!” the Voice said. “This is an official celebration!”

Still, the best moves were for prime spots near the stage — iPhones and Droids at the ready. Everyone looking for their own image, captured in pixels. Just for them.

Before the A-listers took the stage, the crowd contented itself with recorded stuff. Madonna in the house, digitally. The best of the 1990s — it’s the DJ from your cousin’s wedding!

“They threw this party for us,” declared Greg Gershuny, who is celebrating the halfway point of his first year of marriage to his wife, Jessica. He had an old top hat knocking around the house and decided to whip it out tonight. “I figured, if there was ever any place that it would be okay to wear a top hat, this was it.”

Gershuny volunteered in Obama’s first term and went to the youth ball four years ago. He’s all grown up now.

But the food, maybe not so much. The munchies tilted more toward a summer camp vibe than a Michelin-starred joint. Three kinds of pretzels . . . and a pretzel blend! Also: Cheez-Its.

Over at the far end of the airplane hangar, er, ballroom, a large plot of land has been partitioned off for special guests. Between the panels, the regular guests can spot white sofas, gold tablecloths, tea candles. And the special guests are . . .? Whoever, they appear to be feasting on the same Cheez-Its as everyone else.

Bryan Brokmeier, who traveled here from New York for the ball, said it wasn’t quite what he expected. He thought there would be “less concrete on the floor, something other than Cheez-Its” to eat.

“Did you notice there weren’t any chairs?” one woman said to a friend. “I guess we’re expected to stand the entire night!”

Stargazing was the crowd-sourcing sport of choice. But not an easy sport, not on an evening when the VIPs were less overt than on, say, a typical White House Correspondents’ Association dinner.

“Is that Queen Latifah?” one woman asked. (It’s not).

“Is that Keith Urban?” another asked. (Nope!)

Who was here? Richard Schiff (Toby from “The West Wing”) — but isn’t he always? — and Matthew Lesko, a.k.a. the Question Mark man, wearing one of his trademark suits and gamely posing for pictures with fans.

When the guests weren’t ogling, ahem, stars or eating Cheez-Its, there were trinkets to accumulate. Presidential golf balls for 25 bucks, a $50 official inaugural medal, $10 presidential shot glasses.

The official Inaugural Ball was split over separate floors, the better to accommodate about 40,000 guests. And one was decidedly swankier than the other. At the elite level, there were fewer people, more carpeting, and penne al pomodoro instead of pretzels. And . . . Al Franken.

On a night with few real highlights — no president and first lady doing the bump this time — the poor cable networks were left to replay “Obama’s on firrrrre!!” enough times to make a viewer want to scream, “Put out the fire!”

The inauguration night balls are merely the biggest and splashiest in a series of galas that have clogged Washington’s subway system and its taxi fleet for three days with men and women in formal attire. There were unofficial balls to celebrate Kentucky bluegrass, India’s diaspora, Latinos, R&B and on and on . . . and on. There was a Chefs Ball at Art and Soul restaurant to celebrate food and, as it turns out, bipartisanship. “As I say, ‘Fried chicken takes no sides,’ ” said host Art Smith, Oprah Winfrey’s former personal chef. In two weeks, he’ll cook for former first lady Barbara Bush.

None of the alternative balls attracted the president, but they had their moments, too. Some were moments that it took a lifetime to realize. Like when a 94-year-old man walked into the Clarion Hotel in Oxon Hill, where Swing Phi Swing was holding its White and Black Ball. Harvey Lewis has volunteered for every Democratic campaign since John Kennedy’s, and he has been lauded as one of the 2008 Obama campaign’s oldest volunteers.

But he had never been to a ball. Too much money. Never could pull it off. Until Sunday.

An old friend picked up the tab for two tickets, one for Harvey and one for his third wife, Linda.

He pulled her onto the dance floor, swaying to “We are One,” by Frankie Beverly and his soul and funk band, Maze.

“He has not slowed down, he uses no Viagra,” said Linda Lewis, grabbing his hand as he swung her out on to the dance floor. Later, he says with a laugh, he did use Bengay.

“I never felt like such a VIP in my life,” Harvey said.

In the lines at all the balls, at Twitter-trigger speed, people could talk about the day’s viral GIFs. The first lady’s eye roll at lunch with House Speaker John A. Boehner, the Ohio Republican who tangled with her hubby at the edge of the “fiscal cliff.” There was Antonin Scalia’s puffy hat, the one that reminded so many of Sir Thomas More. And there was the photobombing from the day: Bill Clinton photobombing Kelly Clarkson, first daughter Malia photobombing (or was that photo-blocking) as her sister, Sasha, tried to snap a picture of their parents smooching in the inaugural parade reviewing booth.

Layla Crockett could probably relate. She just might be the youngest belle of this particular ball, all of 7 years old. She came up from Huntsville, Ala., to witness history.

“We just wanted her to see something she would always remember,” said her mom, Lisa. “To be a part of something bigger than herself.”

They picked the right night. They picked the right room.

And it’s not as if the president and first lady were pushing Layla too far past her bedtime. Four years ago — before the health-care fight, before the fiscal cliff fight, before hope and change smashed into the realities of sniping and griping in Washington — the Obamas were still dancing past midnight. On this night, the president and first lady were back home just before 10:15 pm.

Katharine Boyle, Jason Horowitz, Maura Judkis, Cara Kelly, Marie Elizabeth Oliver, Robert Samuels, Emily Wax and Dan Zak contributed to this report.