There's something called Seasonal Affective Disorder which is when the winter months bring on depression. It's usually caused by the time shift and gloomy weather. I have Bipolar and I know I get depressed most of the time when it gets to be winter.
That said, just because she's not into Christmas doesn't necessarily mean she's depressed. It could just be that she's growing out of liking it. I don't really much like Christmas, so I can understand where someone could grow out of enjoying it.
If you think it may be a serious case of depression, you may want to talk to her about seeing a doctor or therapist before it worsens.