As Billie Eilish's 21st birthday party approached on Sunday, Kendall Jenner and Hailey Bieber put their hair down and let loose.

During the Christmas bash, the models posed in black-and-white photos with tongues out and middle fingers blazing.

As part of Jenner's Instagram Stories, Jenner shared solo pictures of herself with the three pals, but also a few silly pictures featuring sneers and a kissy face from Justin Bieber.

As Mrs. Claus, Eilish was dressed in a sexy Santa suit that made her look stunning. Gorgios I say....

For her milestone event, she had a three-tiered cake decorated with peppermint candies, as well as a disco ball ceiling hung above a buffet of candy.

It has been confirmed that the guests in Eilish's birthday party raised their glasses in a toast to the "Bad Guy" singer through videos that she shared on her Instagram Stories.

The event was also attended by a few celebrities, including Doja Cat, Avril Lavigne, Noah Cyrus, as well as Hailey's boyfriend, Jesse Rutherford, and Hailey's husband, Justin Bieber.