def _first_of_quarter(self, day_of_week=None):
        """
        Modify to the first occurrence of a given day of the week
        in the current quarter. If no day_of_week is provided,
        modify to the first day of the quarter. Use the supplied consts
        to indicate the desired day_of_week, ex. pendulum.MONDAY.

        :type day_of_week: int or None

        :rtype: Date
        """
        return self.set(self.year, self.quarter * 3 - 2, 1).first_of(
            "month", day_of_week
        )