Mozart's 'Eine kleine Nachtmusik' was written for an entirely unknown occasion

Wolfgang Amadeus Mozart composed 'Eine kleine Nachtmusik' in 1787 — one of the most performed pieces of classical music ever written. Yet there is no record in any letter or document of why he wrote it, who commissioned it, or when it was first performed. Mozart listed it in his own catalogue and then apparently forgot about it. It was not published until 1827, 36 years after his death.

Source: Cliff Eisen, 'Mozart Studies' (1991) / Köchel catalogue K. 525