Advent is, according to the church calendar, the four weeks prior to Christmas. One of the ways many devout Christian families observe the Advent season is with an Advent Calendar, a day-by-day list of prayers, bible passages, and activities.
In Second Life, many popular stores and locations also have "Advent Calendars". These vary in appearance and operation, but the basic idea is that they make a new gift available every day during the Advent season. Many of these require you to visit the location every day, because the gift is only available on its designated day. Others provide a new gift each day, but the previous gifts also remain available for latecomers.
Two of my favorite Advent Calendars are:
Caledon Oxbridge University (go to the koi pond, turn left, then right. Look for the gazebo between the buildings.)