St Nikolaus, just like Advent, is a more continental European tradition that is especially celebrated by children.
Saint Nicholas (“Victory of the people”) is the common name for Saint Nicholas of Myra, who had a reputation for secret gift-giving.
The day before Nikolastag (the night of December 5) children put a boot, called Nikolaus-Stiefel, outside the front door. St. Nicholas fills the boot with gifts, and checks up on the children to see if they were good. If they were not, they will have charcoal in their boots instead. For all the children that weren’t good, Knecht Ruprecht threatened to eat or beat them.
I remember waking up on the 6th December as a child and running to the boot first thing in the morning. Great tradition. :)
So with this, Happy Nikolaus, however you may celebrate it. :D