Mozart completed Symphony No. 40 on July 25, 1788.

The date of completion of the 40th Symphony is known exactly, since Mozart in his mature years kept a full catalog of his completed works; he entered the 40th Symphony into it on 25 July 1788. Work on the symphony occupied an exceptionally productive period of just a few weeks during which time he also completed the 39th and 41st symphonies (26 June and 10 August, respectively).
https://www.classicals.de/mozart-k550

This stamp isn’t a postal stamp, but a Cinderella (word used to for collectable stamps that are not issued by a post office). It was issued to celebrate Mozart’s 200th Anniversary. 

Wolfgang Amadeus Mozart
Austria 1991

No. 40 is arguably the most popular of all of Mozart’s forty-one symphonies, despite the fact that its first movement became one of the most annoying ringtones of the mobile phones of the 90s. It was probably quite popular in Mozart’s lifetime, too. Although scholars can’t be absolutely certain, it would appear Mozart performed it more than once, going on to rescore it for slightly different musical forces.
https://www.classicfm.com/composers/mozart/music/symphony-no-40-g-minor/

A  past SOD from July 25. I actually liked this first flight stamp and historical event, I reposted it from 2024. I almost wrote a new entry for the 1909 event, but thought that might be a bit to geeky. 

Louis Blériot’s flight across the English Channel 1909