Happy Nikolaus-Day! Dec. 6th is Nikolaus in Germany and some other European countries. The children put their boots in front of the door on the evening before, and Nikolaus puts a little surprise in their boots over night. Traditionally it's some tangerines, nuts and sweets, plus a small present.
St. Nikolaus is a historical figure (Bishop of Myra) and the predecessor of your Santa Claus. And I guess your tradition of hanging Christmas stockings (which we don't do) comes from our tradition to put boots outside since they do look more like boots than like stockings.
Our front door this morning: