Fasching, Fastnacht or Karneval are regionally different names for the same festivities, which take place before the Christian fasting time in Germany. Even though the tradition is centuries old, it is still a very popular event for many people in Germany. At Fasching (Carnival) you can see almost everywhere people dressed up in costumes and celebrating happily on the streets.
In pre-Christian times the Fasching celebrations symbolized the banishment of winter and the evil spirits. Today, mainly political events play a role in Fasching. As a protest, citizens mock politicians and parties during the Fasching season, whether in speeches or on the floats during Fasching processions.
Fasching has different names depending on the region. In the Rheinland (Cologne and its surroundings) it is called Karneval, in FrankfurtRheinMain Fastnacht/Fasching and in northern Bavaria Fosnat. Each region has to carnival its own fool call: In large parts of FrankfurtRheinMain one calls "Helau!" in Cologne it is "Kölle Alaaf" ("Long live Cologne!").
While Fasching starts in many regions officially on 11 November, the highlight of the festivities takes place between the middle of February and at the beginning of March. Very popular are the festivities Weiberfastnacht and Rosenmontag. Weiberfastnacht always takes place on Thursday before Ash Wednesday (this year on Thursday 28 February). The day begins by women storming into the town hall and symbolically taking it over. Men are cut off their ties on this day. On the following Saturday and Sunday the big Fasching events and parties take place. Rosenmontag always takes place on the Monday before Ash Wednesday (this year on Monday the 04th of March). This is the big day of parades and everywhere in FrankfurtRheinMain big parades take place. The city of Mainz is particularly famous for its big Rosenmontag parade. The celebrations end on Ash Wednesday (06 March) and the Christian Lent period begins.
You can read what, when and where takes place on the websites of your city. A good overview can also be found in the calendar of events on the region's tourism page.