A traditional Advent calendar has 24 doors, one for each day from December 1 through Christmas Eve on December 24. Opening one door daily creates a countdown to Christmas.
The format grew out of German Advent customs, especially among German Lutherans in the 19th and early 20th centuries. Early calendars could use chalk marks, pictures, or candles rather than hinged doors. Mass-produced versions eventually placed images, Bible scenes, poems, chocolates, or small gifts behind numbered panels.
The number 24 reflects the common popular calendar, not the exact length of the church season. Advent begins on a different date each year—the fourth Sunday before Christmas—and can last 22 to 28 days. Consequently, some calendars begin on Advent Sunday, include December 25, or continue to Epiphany. Still, the familiar traditional format has 24 doors.