The Hearthstone December is of course all about the coming expansion Kobolds & Catacombs. Accordingly, the back of the card for the December ranking season is kept in the treasure hunt style:

To get this card back you just have to Rank 20 in one of the Hearthstone leaderboards. Whether it is standard format or in wild mode, it does not matter. Up to rank 20 you don't lose any stars if you lose.

Hearthstone Ranking Rewards

There is also a loot box with rewards in December:

"The loot from the reward chest for the rated mode depends on your highest rank achieved within a rating season. Reward Chests can contain Seasonal Card Backs, Arcane Dust, or Gold Cards and are awarded to you when you play Ranked Mode in Standard or Wild. Each rating season you will receive a chest that depends on your highest rank (regardless of the format) "

Not only the content of the box depends on the rank you have achieved, the starting position for the next season is also influenced by it:

“Remember, the higher your rank in Ranked Mode, the more bonus stars you will receive at the start of the next season. In the rated mode you will receive bonus stars based on your performance in the past season - one bonus star for each rank achieved. Please note that you have to log in at least once in the relevant rating season, even if you have achieved rank 20 or better with your bonus stars. Only then will your progress count towards the back of the card. "