The idea is a winner: You get a flat, holiday-themed box, and once a day for the 24 days leading up to Christmas, you get to open one of the little doors on the front. Here's where it unravels: Behind the door — at least in most cheap, modern Advent calendars — is a piece of "chocolate," but the low-grade kind manufacturers aren't legally allowed to call chocolate (there's a heavy emphasis on those quotation marks). It's a real let-down, especially given how fun the premise is.
Years ago, though, Nintendo and German developer tons of bits mastered the art of the Advent calendar with UPIXO In Action: Mission in Snowdriftland.
On December 1, 2006, Nintendo of Europe published Mission in Snowdriftland, a Flash platformer game, online. Rather, it published the first level. It was an Advent calendar, after all, so just one level went live on the 1st, then another the next day, and so on until Christmas Eve.
The game was well above the standard of most Flash games of the era. Chubby Snow, the adorable playable snowman protagonist, controls comfortably, albeit with some slippery physics, but those are easy to get used to and suited to the mostly wintery environments. The levels were also relatively large and fun to explore, especially if you took on the extra challenge of collecting all 24 snowflakes in each stage.
There was even a plot: An evil penguin, El Pix, stole game files from the human world and brought them to Snowdriftland. The situation puzzled UPIXO (the tragically fictional United Pixelheroes Organisation), who didn't have any heroes able to survive the frigid conditions of Snowdriftland. Coincidentally, that's when they happened upon Chubby, who took on the task of retrieving the game files and eventually defeating El Pix (on Christmas Eve, per the Advent calendar structure).
The game was a win-win for both tons of bits and Nintendo. To collaborate with an industry titan was a tremendous opportunity for the developer. For Nintendo, the game served as an easy pre-holiday advertising platform: The game files you'd recover for beating the levels promoted DS and Wii games, while Nintendo game images had their own window next to the level as you played. For players, it was also just a fun (and free) daily thing to look forward to for a few weeks, to help build anticipation for the holiday season and make video games a bigger part of it.
But, as might be expected given the game's structure, it was short-lived. It was playable for a bit longer than the initial 24-day window, but it eventually went offline on December 16, 2007. Aside from a December 2010 re-release, the game was unplayable for years and considered lost media.