// Date picker overriden
public class RangoDatePicker extends DatePicker {

    private final static DateTimeFormat monthFormat = DateTimeFormat.getFormat(PredefinedFormat.YEAR_MONTH);

    public RangoDatePicker(int month, int year, RangoDatePickerGroup rangoDatePickerGroup) {
	super(new MonthSelector() {
	    private Grid grid;

	    @Override
	    protected void refresh() {
		String formattedMonth = getModel().formatCurrentMonth();
		grid.setText(0, 0, formattedMonth);
	    }

	    @Override
	    protected void setup() {
		// Set up grid.
		grid = new Grid(1, 1);

		CellFormatter formatter = grid.getCellFormatter();
		formatter.setStyleName(0, 0, "datePickerMonth");
		formatter.setWidth(0, 0, "100%");
		grid.setStyleName("datePickerMonthSelector");
		initWidget(grid);
	    }
	}, new DefaultCalendarView(), new CalendarModel() {
	    @Override
	    public String formatCurrentMonth() {
		return monthFormat.format(getCurrentMonth());
	    }
	});

	// style
	addStyleName("rango-cal");

	// group
	rangoDatePickerGroup.addRangoCalendar(month, year, this);

	// set current month
	Date currentMonth = new GregorianCalendar(year, month, 1).getTime();
	this.setCurrentMonth(currentMonth);
    }

}

//Date picker group
public class RangoDatePickerGroup {
    static HashMap<String, RangoDatePicker> datePickers = new HashMap<String, RangoDatePicker>();

    public void addRangoCalendar(int month, int year, RangoDatePicker rangoDatePicker) {
	datePickers.put(key(year, month), rangoDatePicker);
    }

    private String key(int year, int month) {
	return year + "-" + month;
    }

    public void selectDates(Date start, Date end, int colorCode) {
	for (final Date date = start; date.compareTo(end) <= 0; CalendarUtil.addDaysToDate(date, 1)) {

	    @SuppressWarnings("deprecation")
	    /* Yes, it's deprecated */
	    final int month = date.getMonth();
	    final int year = date.getYear();

	    datePickers.get(key(year, month)).addStyleToDates("rango-colour"+colorCode, date);
	}
    }

}

//CSS

.rango-cal .datePickerDay.datePickerDayIsFiller{
	visibility: hidden;
}
.rango-cal .datePickerDay.datePickerDayIsWeekend{
	background: transparent;
}
.rango-cal .datePickerDay.datePickerDayIsValue{
	background: transparent;
}
.rango-cal .datePickerDay.rango-colour-0 {
	background-color: green;
}