While it is confirmed for December, it won't be the same day as Xenoblade Chronicles 2. I forsee it coming maybe the Week before Christmas as the Holiday Special for that month as the Mario + Rabbids Kingdom Battle Gold Edition was for Thanksgiving this month.
Where did you hear that rumor? And for people who have said it is confirmed for December no it is not. All they have said is around the holidays and we are around the holidays. But considering we are getting towards the end of November with out it most likely it will be December.