But on Saturday, there was nothing bitter about the "Your Body" songstress' surprise birthday bash.
Aguilera headed out of her Beverly Hills home to run holiday errands and do a little Christmas shopping Saturday, a source told Us Weekly. In the meantime, boyfriend Matt Rutler and a few close friends quickly set up decorations for the surprise party.
Upon her return home, Aguilera found 20 of her closest pals waiting for her, as well as a two-tiered black, pink and white cake from Vanilla Bakeshop.
"There were tons of balloons throughout the house," the source said of the party. "Christina was completely surprised. She was very happy."
Guests to the fashionable fete - Aguilera's home was decorated in the same shades to match the cake - included Nicole Richie and Blake Shelton (wife Miranda Lambert was out of town), and featured a smorgasbord of finger foods, including mac n' cheese, mini chili dogs, deviled eggs and grape leaves.
Even Aguilera's son Max, who will turn 5 in January, was on hand to enjoy the festivities.
The proverbial icing on the cake?
Aguilera's friends tested out their pipes with a rousing rendition of "Happy Birthday" when the birthday gal walked through the door.
"Everyone was very friendly and got along well," the source said. "They had a great time."
On Tuesday, Aguilera kept the party going at the afterparty for The Voice finale, donning a Santa hat, a ripped Mickey Mouse T-shirt and fishnet stockings.
"Thank you #Fighters for the many sweet birthday messages today!" she tweeted. "Let There Be Love to you all! XoXtina."
This article originally appeared on Usmagazine.com: Christina Aguilera's Birthday: The Bash, Cake and More Revealed!