Our group handles information used for clinical care in a hospital, so limiting Friday and late-in-the-day installs to exceptional cases, and having easy backouts available are both essential rules. It's harder on weekends and nights to contact the software staff needed to diagnose and fix something if bugs surface sometime later, and the staff using the software on off shifts is smaller and less well informed about changes.

We usually avoid Mondays because they can be crowded with handling questions and issues from the weekends, but we will sometimes choose a Monday install if we want things running for a full 5 days with full staff on-hand.