code: private void labelIndicatorMouseEntered(MouseEvent e) {
          // Skip this function if the settings have not been applied.
          if (settings == null) {
              return;
          }
          JLabel label = ((JLabel) e.getSource());
          // Do not highlight the today label if today is vetoed.
          if (label == labelSetDateToToday) {
              DateVetoPolicy vetoPolicy = settings.getVetoPolicy();
              boolean todayIsVetoed = InternalUtilities.isDateVetoed(vetoPolicy, LocalDate.now());
              if (todayIsVetoed) {
                  return;
              }
          }
          // Do not highlight the month label if the month menu is disabled.
          if ((label == labelMonth) && (settings.getEnableMonthMenu() == false)) {
              return;
          }
          // Do not highlight the year label if the year menu is disabled.
          if ((label == labelYear) && (settings.getEnableYearMenu() == false)) {
              return;
          }
          // Highlight the label.
          label.setBackground(new Color(184, 207, 229));
          label.setBorder(new CompoundBorder(
              new LineBorder(Color.GRAY), labelIndicatorEmptyBorder));
      }
docs: labelIndicatorMouseEntered, This event is called when the user move the mouse inside a
  monitored label. This is used to generate mouse over effects for the calendar panel.