When an opera performance starts, the overture is the first thing the audience hears, but it is the last thing the composer writes. Rossini disliked writing overtures, and the various impresarios he worked for had legendary difficulties keeping him on track. I haven’t found why Mozart waited so long to compose the overture to Don Giovanni, but for whatever reason, it produced a drama equal to anything Rossini did.
Don Giovanni received its first performance in Prague, and Mozart had to travel there for the rehearsals. After the dress rehearsal, that is, the night before the opening performance, Mozart decided to pull an all-nighter and write the overture and asked his wife to stay up with him.
Constanaza’s job was to make punch to keep his spirits up and keep talking to him. So she told him stories from Arabian Nights and other common collections of stories. Probably only Mozart could ever listen to stories, laugh at them, and compose music at the same time.
The trouble was that the punch made him even sleepier than he would have been ordinarily at that time of night. Whenever his wife stopped talking, he began to doze off. Even Mozart could not write music in his sleep. Finally, she suggested that he take a nap and promised to wake him in an hour.
He slept so soundly that she didn’t have the heart to keep her promise. She let him sleep for two hours. By that time, it was five o’clock in the morning. He finished composing at seven and delivered the score to the copyists.
By the time the copyists finished writing out the parts and passed them out, there was no time for the orchestra to rehearse the overture. So they sight-read it. They must have played it pretty well. One of the members of the orchestra later wrote that the overture roused the audience to great enthusiasm. Mozart turned to the orchestra and said, “Bravo, bravo, gentlemen. That was excellent.”