Tram lines 20 and 21, including nightline N20, will be shut down completely in summer 2008 (July and August) to allow for massive track renewal work in Dachauer Straße. There will probably be a bus service instead, slowly working its way to the then-jammed Dachauer Straße.

Stadtwerke München has now published the dates on which tram lines 20 and 21 will be shut down:
May 13th through May 25th, 2008
August 4th through September 15th, 2008
which is during the Pfingst and summer school holidays.

Source: Stadtwerke München bid invitation for contractors: *klick*