Modern Advent calendars are most accurately described as surprise gift boxes. Each Advent calendar, regardless of size, is divided into 24 sections, each covered with a small cardboard door and hiding some kind of a treat. The treat is most commonly a piece of chocolate (though cookies, wafers, and even jams, cheese, and beverages have long become common).
Advent calendars were invented by German Lutherans sometime in the early 19th century. At first, they were heavy wooden constructions decorated with biblical images. In 1908, German publisher Gerhard Lang started mass-printing cardboard Advent calendars, adding the element of surprise by hiding each biblical image under a tiny door.
It’s generally agreed that the first mass producer who added chocolate to Advent calendars was British giant Cadbury, either in 1958 or 1971. According to specific sources, smaller manufacturers tried doing it earlier but failed to amass enough sales to make a solid footprint in history.
Nowadays, Advent calendars with some sort of treat hidden behind the doors are nigh unimaginable. Multiple confectionery giants like Italian Borgo de Medici, German Milka, and Irish Butlers consider Advent calendars a mandatory element of their Christmas product line-up.