code: public void setWeekNumberRules(WeekFields weekNumberRules) {
          if (weekNumberRules == null) {
              weekNumberRules = WeekFields.of(locale);
          }
          this.weekNumberRules = weekNumberRules;
          zDrawIndependentCalendarPanelIfNeeded();
      }
docs: setWeekNumberRules, This sets the week number rules that will be used to show the week
  numbers (the weeks of the year), whenever the week numbers are displayed on this calendar.
  
  The definitions for the first day of the week, and for the minimum number of days that
  constitutes the first week of the year, varies between locales. The Java time WeekFields
  class specifies those rules.
  
  If you set this setting to null, then the weekNumberRules will be set to match the
  DatePickerSettings locale. By default, the week number rules will match the
  DatePickerSettings locale.
  
  If you would like the week number rules to be the same for all calendars (regardless of the
  locale), then you should consider setting the weekNumberRules to one of these two widely used
  rule sets: WeekFields.ISO or WeekFields.SUNDAY_START.