’Twas 1 p.m. on Sunday, and all through the National Building Museum, hundreds of creatures are stirring — just not Justin Bieber, who’s off dreaming about sugarplums.

The Biebs has blown off rehearsal to sleep in, putting some good, old-fashioned holiday panic into the usual clockwork preparations for “Christmas in Washington,” the annual pop concert taped downtown.

Along with the sleepy teen superstar, this year’s bill includes Oscar winner Jennifer Hudson, increasingly ubiquitous country trio the Band Perry, Nickelodeon star Victoria Justice and Cee Lo Green of “[Forget] You” fame, and is hosted for the first time by Conan O’Brien. Among the 999 invited guests in attendance: President Obama, the first lady, the first daughters and the first grandma, Marian Robinson.

Showtime is less than six hours away, and Green is just arriving. He plops down in a front-row seat labeled “POTUS,” nursing a mug of something hot. Once he’s called to the stage, he opts to save his voice, mumble-crooning “This Christmas” as if he’s just returned from the dentist. (He’s actually just returned from a last-minute tux-fitting at Rochester Big & Tall.)

Executive producer George Stevens Jr. (amber sunglasses, cable-knit sweater the color of juiced grapefruit) and producer-director Michael Stevens (casual blue jeans, serious tan blazer) look concerned. The father-son team are fresh off producing last Sunday’s Kennedy Center Honors, another TV extravaganza populated with musicians that follow the duo’s “a little something for everyone” credo.

“This show, like all music shows, really has to evolve with the way the music business is evolving,” Stevens-the-younger says at rehearsal. With the music business ever slumping, that means latching onto hyper-telegenic stars with onscreen magnetism — be it on Nickelodeon (Justice), on “The Voice” (Green), in an Oscar-winning role (Hudson) or in a 3-D concert film (Bieber). They also have to be able to really sing.

“Christmas in Washington” is celebrating its 30th anniversary, as well as 30 years of support for the Children’s National Medical Center. O’Brien mentions this during the run-through and later explains the true meaning of Christmas: “We celebrate the arrival of a miracle child worshipped by millions around the world . . . Justin Bieber.”

Then a woman who is definitely not Bieber appears, singing the teen’s new reggae-tinted single, “Mistletoe,” with gusto. When’s the kid gonna show up? Bieber made his TV debut at “Christmas in Washington” two Decembers ago, when few knew his name. The president accidentally called him “Justin Bye-ber.” Twenty-four months, millions of albums and a few inches in height later, he’s at the highest heights of pop popularity.

So no pressure, 18-year-old Victoria Justice, as you prepare to release your debut album next year. “I’m a little nervous,” she says backstage in a tone that indicates she’s not nervous at all.

Not the case for Hudson. The Chicago native says she’s “lost count” of how many times she’s performed for the Obamas, “but every time feels like the first time, and I get anxious.” And she didn’t want to miss this opportunity: “I always watch this at home on the TV and [I’m] like, ‘Why aren’t I there?’ ”

The odds of the Band Perry missing the show were unlikely, considering their superhuman ability to materialize at televised events like this. In the past few weeks, the sibling country trio has appeared at the White House, the American Music Awards, a Grammy-nomination-announcement concert, the American Country Awards and the Country Music Association Awards.

Tonight, they’re doing Bing Crosby’s “I’ll Be Home for Christmas,” a song that they are trying to make come true. “Unless something comes up,” says Reid Perry.

When Bieber strolls into his dressing room shortly after 3 p.m., the sighs of relief are actually audible. He’s got girlfriend Selena Gomez on his arm and a smile on his face — a nonchalance he didn’t possess the last time he was here. Bieber said his “Christmas in Washington” debut two years ago was also the first time he ever felt nervous about performing. Surely, he’s become more familiar with butterflies since then, gracing “Saturday Night Live” and the Grammy Awards.

“Not really, no,” Bieber says later in the greenroom. “It’s been kind of smooth sailing after that. Nothing is as nerve-wracking as performing for the president.”

By 6:45, the Obamas have taken their seats, along with Vice President Biden, Jill Biden, Sen. Tom Harkin, Wolf Blitzer and the like. O’Brien is hilarious. Green is wailing. Bieber is a charmer. The Perrys are graceful. At one point in her performance, Hudson clutches at her sequined gown as if her gale-force pipes are wassailing her somewhere against her will.

A full orchestra and three sprawling choirs have their back: the Washington Youth Choir, the American Family Choir and the U.S. Naval Academy Glee Club, which opened the show singing, “Christmas is the perfect time to be in Washington, D.C.”

On paper, those lyrics sound ridiculous. Is anyone passing eggnog across the aisle this December? But in the halls of the National Building Museum, with great singers singing great, the words actually feel true.

Christmas in Washington

airs on TNT on Friday at 8 p.m.