Hmm … well, the last time the eight major planets of the Solar System appeared in the night sky together was AD 949. The next time it will happen is 6 May, 2492.
471 years is a bit too long to spend on a single tune, even for elektronauts, so how about 6 May, 2021 instead? That is 6 May, 2021 0:00 GMT (Greenwich mean time).