With the opening of each little door, Christmas Eve creeps closer. Advent calendars aren't just popular for children in Germany, but also adults. Whether purchased or handmade, filled with chocolate or toys, they've got a big fan following. The story of this German tradition traces its roots back to early 20th-century Protestantism.

Until the 16th century, children received their holiday presents on St. Nicholas' Day, December 6. Reformist Martin Luther, however, rejected the veneration of saints and changed the tradition. Since then, gift exchanges occur on Christmas. To shorten the longer waiting time for children, Protestants developed several customs, including the Advent calendar.

Poor families often just marked 24 chalk lines on the door, and the children would erase one each day. Others put straws in a manger. Variations included candles with 24 tick marks or paper chains, from which one link was torn off each day. In some more wealthy homes, the children got gingerbread. Even Catholics enjoyed the advent calendar, and the tradition spread across Germany.

In 1902, a Protestant book store in Hamburg printed the first Advent calendar - in the form of a clock. Two years later the newspaper Stuttgarter Neuen Tagblatt included a "Christmas calendar" in one of its editions. In 1908, the Munich publishing house Gerhard Lang sold colorful photos to cut out and paste onto 24 slots on cardboard.

The first Advent calendars with little doors made their debut on the market in 1920. Behind every door was a picture or Bible verse. During Nazi rule, fairytale figures or Germanic gods replaced the Christian symbols in order to separate Christmas from its religious background.

From the 1950s onward, the Advent calendar became affordable as a mass-produced product. Behind the doors, little photos with snow-covered villages and landscapes or religious motifs could be found. Some calendars contained chocolates or other sweets. The calendar pictured here is from 1965.

Most of today's store-bought Advent calendars contain chocolates in various holiday shapes. Even little toys or other tiny treasures could be hiding behind the door. For those who choose to make their own Advent calendar for loved ones, anything goes. For example, 24 little wrapped packets representing each day could be tied onto a string or put into a box.

Christmas is no longer just a Christian holiday, but also a festival of consumerism. There are now Advent calendars around the world featuring jewelry, beer, cosmetics and other goods. In some cases, the motifs are universal: Instead of the Christ child, there are chocolate figurines in the shape of bears and even Bambi.

Meanwhile, some cities feature house-sized Advent calendars, where a real window is opened every day. The world's largest free-standing Advent calendar is part of a house in Leipzig. Its area measures 857 square meters (9, 200 square feet). Up until Christmas Eve, one of the windows opens at 4:00 p.m. each day to reveal an illuminated holiday motif.