The reason people stand at Lambeau is that the aluminum seats are too cold to sit on. To those wanting to venture to Lambeau in late December, go for it. I hope you enjoy your trip. But you better start looking at hotel rooms now--there are not many in the Green Bay area. I just checked Green Bay hotels, and they are sold out except for a property called the Aloft with rates at $500 per night.
The two times I went to a game in Green Bay I stayed in Appleton, and there is a Rally bus that goes to the game from one of the local bars. You can sign up online. Just keep in mind it is a school bus--it is not a comfortable trip, especially if you have drinking at the bar. It does park only nine blocks or so from the stadium.
And the game is scheduled for December 23, which means you will be flying back on December 24 or 25. On Delta, the cheapest flights to and from Green Bay on Christmas Eve are over $700 and on Christmas Day are over $600.
The NFL could not have picked a worst date for the Saints to play in Green Bay.