Here's a fun graph for you: number of christmas albums released per year since 1940. Starts off fairly low, then, starting in the early nineties, the numbers begin to skyrocket, peaking at 70(!!) in 2008, with the numbers declining heavily since then. I wonder what's up with that? I mean, Mariah Carey didn't help, but she alone can't be responsible for that peak a decade and a half later, can she? The world may never know...
In case you're curious, all data basically came from the wikipedia categories for " Christmas albums". That probably misses some albums, but it's close enough I'm sure.