code: def previous_session_label(self, session_label):
          
          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]
docs: 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.