code: private boolean isWorkingDay(ProjectCalendar mpxjCalendar, Day day)
     {
        boolean result = false;
        net.sf.mpxj.DayType type = mpxjCalendar.getWorkingDay(day);
        if (type == null)
        {
           type = net.sf.mpxj.DayType.DEFAULT;
        }
  
        switch (type)
        {
           case WORKING:
           {
              result = true;
              break;
           }
  
           case NON_WORKING:
           {
              result = false;
              break;
           }
  
           case DEFAULT:
           {
              if (mpxjCalendar.getParent() == null)
              {
                 result = false;
              }
              else
              {
                 result = isWorkingDay(mpxjCalendar.getParent(), day);
              }
              break;
           }
        }
  
        return (result);
     }
docs: Used to determine if a particular day of the week is normally
  a working day.
  
  @param mpxjCalendar ProjectCalendar instance
  @param day Day instance
  @return boolean flag