An advent calendar is a special calendar used to count down the days leading up to Christmas. It typically consists of small doors or flaps that can be opened each day during the month of December, revealing an image, poem, or message behind it. The number of doors corresponds with the number of days until Christmas Day. Advent calendars are often associated with children and families, but they have become popular among adults as well. They serve as a fun way to celebrate and count down to the holiday season.