The German Union Verdi has announced a strike on Wednesday 27 April for the airports Frankfurt/Main, Munich, Dusseldorf, Cologne/Bonn, Dortmund and partly also Hannover-Langenhagen for ground handling, check-in and security services, as well as a section of the fire brigades.
On Wednesday 27 April, flight delays and cancellations may be expected for all airlines operating at these airports.
http://www.lufthansa.com/de/en/Travel-information