def update_previous_events(calendar_event, pre_save, post_save)
    previous_event = @calendar.terms_and_holidays.find_by(end_date: calendar_event.start_date_was - 1.day)
    if previous_event
      callback = calendar_event.start_date > calendar_event.start_date_was ? post_save : pre_save
      new_event_end = calendar_event.start_date - 1.day