Rauhnacht is the period between the winter solstice on 21st December and the Feast of the Epiphany on the 6th January, when the Devil and the souls of the dead make mischief on Earth.
Traditionally, villagers dress themselves in ghostly or macabre costumes and process through the village to drive out the demons through a combination of noise and their ghoulish appearance. The tradition of setting off fireworks on New Year's Eve also has its origins in these "Perchten" processions, when people would use the noise to drive out the evil spirits.
The etymology of the name is not entirely clear but may lie in the tradition of people burning incense in their houses to cleanse them of the evil spirits - Rauch being the German for smoke. An alternative theory is that the name derives from the German word Rau (rough) and is a reference to the hairy demons, dressed in animal furs that are abroad during this time and keep people awake in fear.
Today, the tradition has much in common with the way that the Americans celebrate Halloween and the costumes that people wear are often meticulously prepared in advance of the Perchten processions. Another similarity with the Halloween celebrations is that children go from house to house seeking treats. In the past it was doughnuts but today it is more likely to be sweets.