Given a session label, returns the label of the previous session.

        Parameters
        ----------
        session_label: pd.Timestamp
            A session whose previous session is desired.

        Returns
        -------
        pd.Timestamp
            The previous session label (midnight UTC).

        Notes
        -----
        Raises ValueError if the given session is the first session in this
        calendar.

def previous_session_label(self, session_label):
        """
        Given a session label, returns the label of the previous session.

        Parameters
        ----------
        session_label: pd.Timestamp
            A session whose previous session is desired.

        Returns
        -------
        pd.Timestamp
            The previous session label (midnight UTC).

        Notes
        -----
        Raises ValueError if the given session is the first session in this
        calendar.
        """
        idx = self.schedule.index.get_loc(session_label)
        if idx == 0:
            raise ValueError("There is no previous session as this is the"
                             " beginning of the exchange calendar.")

        return self.schedule.index[idx - 1]