On googling round a bunch of German sites unsuccessfully looking for the image CS mentioned, I came across quite a few references to the idea that the Virgin was often identified with the harvest/grain goddess Ceres. The main festival of Ceres was the Eleusinian Mysteries, same sort of time of year as we're talking about here.

This doesn't mean in any way that mediaeval people celebrating the Assumption would have "really" been celebrating a harvest festival covertly, but it may explain why the Assumption got moved to its present date.