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This is about Friday, September 24, 1869. It is known as "Black Friday" in the US because of a failed ploy that left many wealthy investors broke. The investors tried to corner the market on gold, buying as much of it as they could and driving up the price, but when the government found out, they released $4 million worth of gold into the market, driving down the price and clobbering the investors.
While the song is about events in the US, it mentions a town in Australia: "Fly down to Musswellbrook." Musswellbrook is a rural town 2 hours North of Sydney that is full of kangaroos (thus the line, "Nothing to do but feed all the kangaroos"). It's possible that Walter Becker and Donald Fagen selected the name of Musswellbrook from an atlas, mainly because it worked well with the next line, "I'm going to strike out all the big red words from my little black book." They also wanted a place far away from Los Angeles. (thanks, Adrian - Sydney, Australia)