This is a sample and useful customization. It's used to show holiday dates with other color and with a tooltip showing the holiday name. You can use the factory class {@link net.sf.nachocalendar.holidays.HoliDayCalendarFactory} to create {@link net.sf.nachocalendar.components.DateField}, {@link net.sf.nachocalendar.components.DatePanel} and {@link net.sf.nachocalendar.components.CalendarPanel} components customized this way.