Short answers, pulled from the story.
Mozart completed Symphony No. 39 in E-flat major, K. 543, on the 26th of June 1788. It was the first of three symphonies he finished in rapid succession that summer, followed by No. 40 on the 25th of July and No. 41 on the 10th of August.
It cannot be established whether Symphony No. 39 was ever performed during Mozart's lifetime. The earliest known eyewitness account of a performance comes from an all-Mozart memorial concert in Hamburg in March 1792, more than a year after Mozart died.
Iwan Anderwitsch attended a Mozart memorial concert in Hamburg in March 1792 and described the symphony's opening as so majestic it prevented even the coldest listener from being inattentive. He wrote that the music was nearly impossible to follow so rapidly and that some audience members admitted they had never imagined hearing anything performed so splendidly in Hamburg.
The autograph manuscript, part of a volume containing nine Mozart symphonies in his own hand, sold at Sotheby's in London in 1987 for 2.5 million pounds, equivalent to approximately 4.7 million dollars. That price set a record at the time for a non-medieval manuscript sold at auction.
The trio section of Symphony No. 39's third movement is an Austrian folk dance called a Ländler. It features a clarinet solo and a second clarinet playing arpeggios in its low chalumeau register. The melody derives from drinking songs that were popular in Vienna during the late 18th century.
Yes. Conductor Nikolaus Harnoncourt argued that Mozart composed Symphonies Nos. 39, 40, and 41 as a single unified work rather than three independent pieces. He pointed to the fact that Symphony No. 39 opens with a grand overture-like introduction but ends without a coda as evidence of this design.