I would imagine it is Monday for music because of the weekly chart, which allowed the full week of sales before the next Sunday chart.
Friday for games may have been to seperate it from music, but could also have been as it is easier to control early release. If you release a game on Monday, you have to ship to stores on Friday/Saturday, therefore they can sell them early, whereas Friday release, can be shipped on Thursday, stopping anyone selling early. There is always problems with this, but it is only a theory.