NEW YORK — Christian Siriano is in a celebratory mood.

And rightfully so: The designer, who though a veteran still feels in some ways like an underdog (or at least someone who feels an urgency to dress them), marked his 15th anniversary in the biz with a star-studded front row and one surprise performance Friday afternoon, kicking off the first official day of New York Fashion Week in a flowering ballroom at the Pierre Hotel.

A major moment came just minutes before the show began, as Janet Jackson made her grand entrance. The singer, who has taken in Siriano's show from the front row before, arrived fashionably late surrounded by hulking bodyguards who escorted her to her seat amid a rumble of excitement from attendees.

The spring/summer 2024 collection had a festive outlook, featuring sparkling confetti dresses, black tie ensembles with an edgy twist, sparkling sheers and the return of the bubble skirt silhouette.

Jackson, more agog over each design coming down the runway than the one before it, mouthed "that's beautiful" and "I love that" for multiple looks, including a black-and-white ensemble with sheer elements and a structural tutu design early in the show. Like a proud stage mom, the iconic singer whipped out her phone to snap photos and videos as models including Coco Rocha, Lauren Chan and Candice Huffine strutted by.

But Jackson wasn't the only singer to shock the crowd: Sia made a special cameo to perform as the show came to a close.

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Avril Lavigne, Kesha, Rosie Perez sit front row at Christian Siriano's New York Fashion Week show

The Siriano front row, always an eclectic mix of his A-list friends and muses, included Jackson, Laura Linney, Rosie Perez, Padma Lakshmi, Laverne Cox, Quinta Brunson, Sarah Hyland, Avril Lavigne, Alicia Silverstone and Kesha.

Linney and Silverstone giggled and held onto one another throughout the show with the physical closeness of friends who have made their bones in the industry for decades (the two also starred together in 2009's "Time Stands Still" on Broadway).

Cox, Brunson and Hyland snapped photos together and talked giddily as the cameras flashed around them before the show started. Lavigne and Kesha, both rocking all-black looks, talked animatedly; Lakshmi captured content of the show on her phone for almost every look.

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Sia performs 'Chandelier,' 'Diamonds' during Christian Siriano finale

The Australian singer-songwriter brought the show to a close, shuffling out in a pink-tiered, cupcake-like dress and a large, bow-adorned pink wig that obscured her face in signature fashion.

As the final models walked in glittering, ribbon-laced and voluminous tulle gowns, Sia belted out her hit "Chandelier." She transitioned into a rendition of "Diamonds," which she wrote for Rihanna, for the finale.

Siriano and Sia hugged and shimmied together when the designer came out for a victory lap after the final model left the catwalk, and the crowd gave him a thunderous standing ovation.

"Thank you to the entire Christian Siriano team and to all of the people who continue to support and wear our clothes over the years. Thank you to all the editors, stylists, photographers, and artists for all the support," Siriano wrote in the show notes.

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