def days_index(df):
    """
    Find indices of daily trips.

    Parameters
    ----------
    df : pandas DataFrame
        Dataframe containing bikeshare trip data with columns that have been
        renamed to the common key.

    Returns
    -------
    d_i : dict
        Contains the indices of the first trip per day.

    """

    days = df['start_dt'].dt.day
    d_i = [(days == i).idxmax() for i in range(1, max(days)+1)]

    return dict(zip(range(1, max(days)+1), d_i))