My first job was as a production lab technician in Ontario.  We made zinc and aluminium alloys.  We had 4 rotating 8hr shifts.  Mon tues off wed thurs off  fri sat off and sat sun off.  One saturday would be an overtime shift composed of the two off shift.  12 hour shifts were often used to make up for days off or if a lot of production required dealing with at the same time. you don't get that flexability with 12 hr scheduled shifts.  There was never any problem getting people to work the overtime.  They had a two week shutdown and christmas