Kept: 30 September
On 30 September 1791, The Magic Flute premiered at a suburban theatre in Vienna, the Freihaus-Theater auf der Wieden. Mozart conducted it himself.
He died two months later, on 5 December. The opera whose last pages he was still writing is the one that never stopped being played.
Kept because the thing you finish last can be the thing that lasts.
(verified: English Wikipedia, “The Magic Flute”)