The check was written from Mitsubishi UFJ to Morgan Stanley in the fall of 2008. This was right when the full extent of the global economic meltdown was discovered, and Wall Street was desperate for capital in order to survive. 

Morgan Stanley got a $9 billion investment from Mitsubishi, which essentially saved the company from collapse at the time. The amount was so large, that you would think that it was to be wired electronically instead of writing a check. There was one little problem.

It was Columbus Day, and banks in the US and Japan were closed. Because it was an emergency, however, Mitsubishi cut an actual physical check that is most likely the largest ever written!