Taylor Swift took to her Twitter to wish best friend Selena Gomez a happy birthday on Monday.

Taylor Swift tweeted: "Happy 21st birthday to my friend, and the closest thing I've ever had to a sister, @selenagomez!!"

Despite her sweet words, Swift wasn't able to make Gomez's birthday bash, Us Weekly reports.

According to the magazine, Selena celebrated her birthday at a French bistro called Bagatelle in West Hollywood with more than a dozen friends, including actresses Ashley Benson and Francia Raisa. The girls posted photos of their soirée on their Instagram accounts.  

"I could not have asked for a better birthday," Gomez tweeted July 22. "This one will be remembered forever."

Her birthday celebrations began earlier on Saturday, July 20. Raisa, Benson, Julianne Hough, and Lily Collins hung out at Beacher's Madhouse in West Hollywood.

"They were all wearing little black dresses," a source told Us Weekly.

Selena also had a special treat for her fans on her big day. She posted a clip of herself partying with her friends to her song "Birthday," which is from her "Stars Dance" album that will come out on Tuesday.

"Selena had a bunch of her friends and her dancer friends come down, and DJ Everette - who is a very prominent DJ in the LA events scene, he does all the big parties - spun her song and they all just danced their asses off for two hours straight," Justin Howard, an events producer and florist who is close to Selena, told HollywoodLife.com.

Selena shot the wild video for "Birthday" in his warehouse in Van Nuys the night of July 18, HollywoodLife.com reports.

"It was so much fun - they all had a blast!" he added.

Watch the "Birthday" song here: