The Serenade for Orchestra No. 9 in D major K. 320, Posthorn, was written by Wolfgang Amadeus Mozart in Salzburg, in 1779. The manuscript is dated 3 August 1779 and was intended for the University of Salzburg's "Finalmusik" ceremony that year.
The above text from the Wikipedia article "Serenade No. 9 (Mozart)" text is available under CC BY-SA 3.0.