Yeah, when I first got the game at Christmas time 2 years ago - I was quite surprised to say the least as to why there were Christmas songs in a racing game. Then as I delved deeper I was tortured by unrelenting Jazz music...everywhere!
It's been like that since GT5 Prologue. They play once your PS3 calender hits the 23rd, and goes all the way until mid-night of the 25th.
Back in GT5 Prologue, you'd get some snow and Santa carrying his presents into a controvertible car (the car varies from region that you're playing the game from)