Advent Calendars are special items that hang on the wall of your pet's home. They are only sold during the Winter Holiday season each year. Buying one of these will give you a free ornament each day that you log in from December 1 - 24. You will only get one ornament for each day that you log in to Pet Society; if you miss logging in for a day, you will not get that day's ornament. As the days go by, your calendar will black out each date that has passed.
Advent Calendars are year-specific: A 2009 advent calendar will not give you ornaments in 2010. You will have to buy a new one each year.