One Staff member does not work on a certain day? Can I set that up?

Under Setup --> Staff you can designate days your employee's work.  Find the Staff in the list, and choose Edit (the pencil icon).  Then scroll down to the Standard Work Days section.

You can also designate a backup Staff that if a task is auto-assigned to the first Staff on a certain day, the system will know to move it to the other.  Note that you can still schedule the first Staff yourself.