Advent calendar

Advent calendars had calendar windows, which slid open. (PROSE: The Ship of Painted Shadows)

Behind the scenes
Advent calendars are used to count the days leading up to Christmas, with each window containing a small gift for the owner of the calendar.

Since 2006, the Doctor Who team has done an "Advent(ure) Calendar" in the days leading up to the Christmas special. Generally beginning on December 1st, and ending on Christmas, when the special airs, each day of the Adventure Calendar includes a treat for fans, related to the series.