The harp is used for many soundtracks. For example the movie soundtrack of Harry Potter, the entire series, the harp is a common used instrument.
The composer is John Williams and he did a fabulous job. The music is such recognizable to so many fans. Even people who haven’t seen any of the movies know the music. And you did something very great when that is the case.
The harp in this case brings a lively Christmas feeling with it that you don’t often see in other music parts. I believe John Williams did a lot with Christmas as his subject for the music. Every time he hits the right notes and uses natural elements to awake a classic feeling within you while listening to his masterpieces.