Sometimes called also the fifth season - The Fasching, also known as Karneval and Fastnacht is the festival season where everyone dresses up - not everyone, it really depends on the region. Generally it is like this: you love it or you hate it. Nearly every town in Germany has a - you guessed it - Verein - where they plan all year for the carnival days. The season starts on the 11. of November at 11.11 am and runs until "Aschermittwoch" - a wednesday in February where it ends. The last day where you party would be the tuesday before that - "Faschingsdienstag". The most appreciated day is "Rosenmontag" - monday where all the large parades are going on in the city centers and everybody parties.
The most known carnival towns are in the middle of Germany, namely Cologne, Düsseldorf and Mainz.
In Cologne they would say "Kölle Alaf", in Mainz "Helau" and in Freiburg "Narri Narro".
You are expected to dress up but not everybody does so. Tuesday is usually the day where all the children have their party, the "Kinderfasching". Loads of cowboys and indians.