An Advent calendar is a special calendar used to count the days of Advent in anticipation of Christmas. Since the date of the First Sunday of Advent varies, falling between November 27 and December 3 inclusive, many Advent calendars, especially those that are reusable, often begin on December 1, although those that are produced for a specific year often include the last few days of November that are part of the liturgical season. The Advent calendar was first used by German Lutherans in the 19th and 20th centuries.
We always had an Advent calendar when my sister and I were kids and we would take turns fighting over who would get to open the door and eat the piece of chocolate behind it. It was a tradition that has really caught on in recent years and now you can find Advent calendars for almost anything.
Here are our Advent calendars this year:

I have two cheese ones. If I had purchased a wine Advent calendar life would have been perfect! :-)
The two chocolate ones are for Paul.
Of course we bought Advent calendars for the cats. We hadn't intended on buying two but you can see from the one on the right happens when I buy a package containing freeze-dried liver bits and don't hide it away immediately. I know, bad Jerry. In my defence, I had to go to the bathroom really badly. By the time I had finished washing my hands the damage was done. Of course I had to buy a replacement which might have been their Machiavellian plan all along.