An advent calendar is a special calendar used to count down the days until Christmas. Traditionally, it starts on December 1 and ends on December 24, the day before Christmas Eve. Each day features a small door or window that can be opened to reveal a treat, such as a piece of chocolate or a small gift. Advent calendars originated in Germany in the 19th century and have since become popular worldwide. They often include religious themes, reflecting the anticipation of the Christmas season, but many modern versions focus on fun and festive surprises for children and adults alike.