I would suggest that Friday makes a good time for the mandatory day. Means people can go out for drinks at the end of the day. The in office days will never be as productive but they can be good socialising times so you may as well make use of that.
I'm also in favour of Thursday as the mandatory day if I need to be in the office as the last day of the week to do release management, e.g. minimise the risk of a bad deployment on a Friday affecting someone's weekend.
1: This may be a Dutch phenomenon, but many shops and amenities are closed on Monday morning. Especially with the gradual increase of shops being open on Sunday afternoons.
This is the case in France too, it's common for small shops to close Sunday and Monday, and be open the rest of the week.