In accordance with the Federal Government's guideline resolutions of 6 May 2020, each federal state decides for itself when and under what conditions trade fairs may be held again. So it can sometimes be quite difficult to keep an overview of the situation in the individual states.
Our Corona live ticker for trade fairs throws light on the matter. Here you can read about which federal state's trade fairs will reopen their doors again when and under what conditions:
2) The German federal states' regulations
Interesting blog posts: