def METHOD_NAME(self, dts: Set[date], multiple: bool = False) -> None:
    """
    When multiple is True, each holiday from a given date has its own observed date.
    """
    for dt in sorted(dts):
        if not self._is_observed(dt):
            continue
        if multiple:
            for name in self.get_list(dt):
                self._add_observed(dt, name)
        else:
            self._add_observed(dt)