On July 25th, 1788, Mozart completed his 40th symphony, K. 550, in G Minor. It's one of only two symphonies he wrote in a minor key, and it's really, really great.
Sixteen days later, on August 10th, 1778, he completed his 41st symphony, K. 551, in C Major. Yep, you heard me. Sixteen days. And it's one of the finest things he ever wrote; a towering masterwork of astonishing beauty and vigor. (Especially the fourth movement; breathtaking.)
Now don't freak out. It's possible that he didn't compose the 41st Symphony in a mere sixteen days; it's possible that he'd already composed it in his head and merely wrote it down in that two week period between late July and early August. That's less freaky, right? (Also, I just realized that he was younger then than I am now. By quite a lot.)
You know what? Go ahead and freak out.