There's a bit of trivia I've remembered about this game and I hope I'm right. If the Saturn clock is set to Christmas day, the games graphics are Christmas themed.
From what I can find, it's actually the demo Christmas NiGHTS into Dreams... that did that. The full game also uses the system time for changes but not that specifically-
The A-Life system features an evolving music engine, allowing tempo, pitch, and melody to alter depending on the state of Nightopians within the level. The feature runs from the Sega Saturn's internal clock, which alters features in the A-Life system depending on the time.
EDIT: Not particularly exciting, but I found and took a picture of my copies - evidently the Christmas demo came from Blockbuster Video.
