Whichever day has the least # of scheduled meetings.
Chiming in on this topic: Never really thought about my most productive day of the week before, so this might be a difficult one to answer: I guess it's Friday, oddly enough, because I'd like to finish outstanding tasks and get things done before the start of the weekend.
@Connor 's answer made me smile.
Btw, I just love Garfield and his Monday mood.
It is always Friday, because on Fridays, I'm in Love....
Mondays tend to be slow to start, but the most productive due to minimal meetings!