def unhighlight_days(self, days):
        for cal in self.cal_ctrls:
            c = cal.GetDate()
            for date in days:
                if date.year == c.GetYear() and date.month == c.GetMonth()+1:

                    # Unselected days either need to revert to the
                    # unavailable color, or the default attribute color.
                    if (not self.allow_future and
                       ((date.year, date.month, date.day) >
                       (self.today.year, self.today.month, self.today.day))):
                        attr = wx.calendar.CalendarDateAttr(colText=UNAVAILABLE_FG)
                    else:
                        attr = wx.calendar.CalendarDateAttr(
                            colText=NORMAL_HIGHLIGHT_FG,
                            colBack=NORMAL_HIGHLIGHT_BG)
                    if date in self.selected_days:
                        attr.SetTextColour(SELECTED_FG)
                    cal.SetAttr(date.day, attr)
            cal.highlight_changed()
        return
Turn off all highlights in all cals, but leave any selected color.
dataset/ETHPy150Open enthought/traitsui/traitsui/wx/date_editor.py/MultiCalendarCtrl.unhighlight_days