The Advent Calendar is only operative in the Month of Celebrating, December. Every day of that month, a new Christmas-related animated video can be seen along with some prizes to go alongside of it. (They include Neopoints and items such as Toys, Food, Books, a Petpet, and Neocash items.) Items will be hidden in each day's animation and reward a wintery-themed bonus prize if discovered.
The Advent Calendar Archives is where you can see all the animations that were showed during all the Calendars. All of the animations from 2000 through 2016, however, were produced using Adobe Flash. They are being converted to a modern movie format so that you can continue to enjoy them each holiday season, even after Flash goes away.