The word ‘advent’ comes from the Latin word adventus or arrival, which itself originates in the phrase ‘come to’. So the whole idea of advent is based on waiting for the arrival of the birth of Jesus in the Christian calendar. In fact Advent is the first season of the Church year officially. It begins on the nearest Sunday to 30 November, and lasts for a period of 25 days.
When the idea of Advent was first introduced by the Christian church, in the 6th century, it was much longer – it actually lasted about six weeks and was considered more a time when you could prepare and purify yourself for the arrival of Christ by prayer and, to some extent, fasting (while never as strict as Lent, in the early church Advent was associated with some restrictions on what you could eat – very different from the chocolate Advent calendars we buy today!) It was also associated with the idea of the Second Coming – the belief that Christ would appear on earth again – so to the earlier Christians it was more than just a celebration of his birth.
But while Advent itself has a long history and has appeared on the Christian calendar since the early years of the faith, the Advent calendar is relatively new. They first began to be used in Germany in the 19th century (it’s interesting how many of our Christmas traditions, such as the tree, originated in Germany). It began as a Protestant tradition, and it’s not quite sure how far back this goes as it wasn’t official; some Protestants in Germany used to start putting chalk marks on their front doors, either 4 Sundays before Christmas or on 1 December (the latter came to be the preferred date, probably because it was easier). They would put one mark for each day until the 25th, and then rub off one mark a day until they were all gone. So this very simple system (and very useful if you didn’t have a calendar in the house, which a lot of people wouldn’t) was the start of the Advent calendar. From this, other traditions soon developed. Some people would put up different religious pictures on different days (a bit like the pictures on Advent calendars today) or others might light a candle.
By the beginning of the 20th century printed calendars were being sold in shops. It’s not sure who invented or produced the first one, but there is a record of a printed Advent calendar in a German newspaper in 1904, and something similar may have been available earlier than that. However, a mass-produced version was definitely available in 1908 – again, this was in Germany – and it was so successful that its maker, Gerhard Lang, kept his business going until the 1930s. These calendars were elaborately decorated and had 24 windows, rather than 25 as is usual today.
Once the first calendar became known, other businesses began to produce their own versions. The features we know today developed quite early, for example the doors with pictures or words inside (though the message with the early calendars was nearly always religious, while now it is more likely to be mainly secular) and even the habit of putting sweets or chocolate inside as a little present for children.
By the start of World War II Advent calendars had become very popular. Production had to stop during the war because paper was rationed (and so were sweets of course), but it started again soon afterwards and today they are among the most popular Christmas decorations.
You can read more about this tradition here.