Matthew Rutler and Christina Aguilera
12/12/2012 at 06:00 AM EST
Just call Christina Aguilera
a Jingle Belle!
coach stepped out on Saturday with boyfriend Matthew Rutler for film and television producer Jennifer Klein's annual private holiday party in Los Angeles.
Arriving around 11:30 p.m. with a small group of friends, the duo "looked to be in the best of moods, sitting in a corner, happily sipping on Russian Standard Vodka Moscow mules while their friends sang and dance to Michael Jackson," an onlooker tells PEOPLE. "They seemed to be quite in love and happy!"
Other guests included Chris Evans and girlfriend Minka Kelly, who donned "giant smiles" as they stayed close throughout the evening.
Meanwhile, Mad Men
's Christina Hendricks dressed for the holiday theme in a red plaid dress and enjoyed cocktails with her husband Geoffrey Arend and friends.
– Jennifer Garcia