I was there last year from December 12-22 and the first time I went she was in her yellow dress, but the second time I went she was in her red dress. I'm not sure of the exact day of the switch, but it is very possible that by the 19th she will have changed outfits.
Edit - After another post saying she was in her yellow dress, I went back to check my reservations from last year, I had breakfast there on December 21 and that was when she was in her red Christmas outfit. Maybe I just got lucky? Or maybe she is only in the Red once
MVMCP ends or something?