There seems to be some availability for the 21st-24th (at ridiculously inflated rates).
If it were me, for the nights of the 20th through the 25th, I'd book right away at the Ritz-Carlton in Wolfsburg - there's availability at €290 as of 13:00 EST . It's just a 35-minute ICE ride to the Hannover Hauptbahnhof, from where you can catch a GVH train to the show. (The Wolfsburg station is right next to the Autostadt complex, of which the Ritz-Carlton is a part).