The glamorous pair ventured out of their sprawling Bel Air mansion to personally pick out a tree – in this case a 10-ft. Noble Fir from Mr. Greentrees in Beverly Hills that Theroux paid for.
"Jen picks her own Christmas tree every year," says a witness. "It's a tradition for her. She's very specific about the tree that she wants. "
At the tree lot, "Jen was in a great mood and took her time to pick out the perfect tree," says the witness. "Justin helped her and she found a beautiful one."
This is the third year in a row that the couple – who have kept a low public profile for the past month – have picked out a Yuletide conifer together.
It's part of a cherished Aniston tradition: her annual tree-trimming party for close friends and associates.
"The tree will be delivered in time for her party," says the witness.
Says an Aniston source about the star's tree-trimming parties, "Jen always has her personal chefs prepare great food for guests, and this is by far her favorite party to host. She always puts in a lot of planning."
"The party will start early in the evening so children can attend," says the Aniston source. "It's always a family friendly event."
That's the perfect celebration for Aniston and Theroux, who got engaged in August 2012 but have weathered tabloid speculation about their relationship in recent months.
"Jen and Justin looked very happy together," says the witness at the tree lot. "She wore her engagement ring and looked amazing."