For me, it's Tuesday or Wednesday, most of the time. Mondays are too overwhelming with catching up on emails, calls, etc. I schedule meetings on Thursdays and Fridays. Tuesdays and Wednesdays are my work through as much stuff as possible days.

What about you?

My most productive day is Thursday. Probably because that is when I get 90% of my work given to me. It has to been done by Friday by 3 at the latest. Monday is always my slowest day. I do spend most of my Monday's going over emails, mail, etc.