Nobody. It's an old saying. On the farm farmers raise their own turkeys. They have turkeys they harvest for thanksgiving and they have turkey they don't harvest until Christmas. The turkeys that are harvested in December have been fed and fattened up for an additional month....thus....they are more full of shit than the ones killed at Thanksgiving.
That's where the expression "full of shit as a Christmas turkey" came from.
I'm surprised you didn't know that, you being a southerner
I'm a big city southerner. I'm not a backwoods redneck southerner.