In the ancient customs of the alpine regions the devil (we call it "Krampus/Tuifl") is a figure of scariness which accompanies St Nicholas in the Advent season.
While St Nicholas gives the wellbehaved kids presents, the devil - the evil opponent of St Nicholas, punishes the naughty kids.
Traditionally, the devil day ("Krampus day"), is on the 5th of December and the festivity of St Nicholas on the 6th of December.
Generally though, both figures appear together in the evening of the 5th of December.