The Happier Than Ever singer certainly knew her references

Billie Eilish rang in her 21st birthday in the biggest way, not only getting to celebrate with her fans while on tour, but also having a bash of her own.

MORE: Billie Eilish's fans obsess over celebrity cameo in latest photo set

The singer hosted a huge star-studded birthday party for herself and many of her friends and family members, and she definitely rocked the occasion with her style as well.

VIDEO: Billie Eilish's poolside message

Several of her celebrity partygoers shared snippets from the occasion, including Billie's Christmas-themed attire, showing up for her party in a mini-dress.

However, what made it special was that it was Santa-themed, featuring a red velvet fit with a white fur trim, a low-cut neckline, a black belt, matching gloves, and a matching cloak too.

TRENDING NOW: GMA3's new anchor talks struggle with getting to host the show

The outfit was very reminiscent of the iconic dresses that The Plastics from wore for their performance, and it was definitely easy for the singer to dance around in.

Model Eve Safdie shared a sneak peek at Billie's outfit and some of the celebrity guests as well, including her pop idol Justin Bieber, who was also partying it up.

Billie partied it up with several of her celebrity friends

Other partygoers included Doja Cat, Olivia Rodrigo, Kendall Jenner, Avril Lavigne, Hailee Steinfeld, and Hailey Bieber.

Billie's boyfriend Jesse Rutherford was right by her side, rocking a Mr. Claus suit to complement his partner's while he helped her ring in the milestone year.

MORE: Billie Eilish opens up about embracing more 'masculine' in her style

ALSO POPULAR: Royal family 2022 Christmas cards from around the world

Before the party came a star-studded show that also featured many of the party's attendees on stage to gush over the singer and support her on tour.

Also in attendance during the show was Donald Glover, whose musical pseudonym is Childish Gambino, as well as singer Khalid, who Billie has collaborated with before.

The singer marked 21 with a hilarious throwback

She was performing the final concert of her three Hometown Encore shows, when she was surprised with a cake brought by her brother, fellow musician Finneas, and her boyfriend.

Read more HELLO! US stories here

Sign up to HELLO Daily! for the best royal, celebrity and lifestyle coverage

ncG1vNJzZmivp6x7qbHLpaammZeWx6q6xGeaqKVfm660tMiopWiblaGyo77IrbBmq6SuuaZ7kWlpa2liZ35ygo9qamlnkp65rbXEZpyipJmotW6yxKyroq6VYrC2vtWeZKGtl5y2r7OMpqCnoV2Zv6a%2F0maZoqqknbGixYybmKygXw%3D%3D