The Advent Calendar is a patch (number 338 on bilious) by Patric Mueller, originally written against NetHack 3.4.3. It has been incorporated into several variants, including UnNetHack, DynaHack, NetHack Fourk, and possibly others.
The patch adds a special level, the Advent Calendar, which is only reachable during the month of December, via a magic portal on one of the early levels in the main Dungeons of Doom branch. When you enter the level the portal is located on, you get the message "you smell chocolate". The portal is only generated in December. The level itself contains 24 small rooms, each protected by undiggable, unphaseable walls and an initially locked artifact door, which is indestructible and cannot be bypassed or unlocked by normal means. Unlike the artifact doors in Slash'em and dnethack, no corresponding artifact key is generated, so the doors cannot be opened until they unlock. Engravings on the ground outside each door number them from 1 to 24. Additionally, pieces of colored glass are scattered around the level for atmosphere, and there are several trees.
Each door unlocks if the level is visited on the corresponding day of the month. Most of the doors contain a minor treat, such as a piece of food; however, the twenty-fourth door contains a "Christmas present", which is typically a more desirable item -- in some cases a magic tool.