Making a schedule "not visible" means that parents can not see the schedule. The schedule does not appear to parents when they are viewing the "schedule list" on the main page. Schedules can be configured to open/close (become visible/not visible) in advance using the Schedule Summary.
"Locking" a date means that the schedule can be seen by parents (unless the schedule itself has been made "not visible), the date & time slots can be seen by parents, but parents are not able to sign up for the date/time slots. Locking/unlocking is done in the Date Summary. There is not a mechanism to configure dates to lock/unlock in advance.
"Not visible" and "locked" do not affect school staff's ability to view schedules or sign up for time slots assuming the staff member is logged in.