Wolfgang Amadeus Mozart’s 261st birthday happily fell on the San Antonio Symphony’s Mozart Festival concert Friday night. The orchestra didn’t play “Happy Birthday,” but it did something better by starting the concert for an audience of nearly 1,250 people at the Tobin Center for the Performing Arts with the composer’s celebratory Posthorn Symphony. Guest conducting was Gerard Schwarz, a director widely recorded with some of classical music’s top stars and ensembles. The Posthorn Symphony is constructed from Mozart’s longer Posthorn Serenade, which was used as graduation music in Salzburg, Austria, in 1779 since Edward Elgar’s “Pomp and Circumstance” didn’t exist yet. Trumpet principal John Carroll expertly played the unvalved posthorn instrument in a solo during the Mozart symphony’s menuetto. The rare instrument happens to belong to Schwarz, a former trumpet player himself.