Lovato, 18, was allowed to leave the treatment facility for the day on Christmas, a source close to her family tells PEOPLE. She returned later in the evening after celebrating with her mom, step dad and two sisters.
"It is the most important holiday to her," the source says.
Lovato checked into the treatment center late October after a fight with a dancer, Alex Welch, on the Jonas Brothers World Tour. What was described as a "short, physical altercation" was settled in time for Christmas, and Welch told PEOPLE she is "ready to move on."
Although there is no date set for the completion of her treatment, a source close to the family told PEOPLE in November that "Demi knows she needs the help and knows that she is where she needs to be right now."