This is Germany's largest open-air festival organised by students, and starts at 5pm on Wed, 25th (today). I've never been before, so can't guarantee if it's any good...
The nearest MVV stop is U6 Studentenstadt. The entrance fee is 2 Euros, which covers all 4 days. There are 5 stages where you can listen to different bands or watch theatre groups, and of course there are beer gardens and international food stalls, with a fire show at 11.15pm each night.
More info (in German only) at www.StuStaCulum.de.
Programme details/map here.