def start_first_monday(df):
    """Slices the dataset keeping data dated after the first monday available

    Parameters:
    -----------
    df : Pandas DataFrame
        the original dataset

    Returns
    -----------
        df : Pandas DataFrame
            the corresponding dataset
    """

    first_date_gap = df.iloc[0].gap_in_day
    next_monday_gap = first_date_gap + (4 - first_date_gap % 7) % 7
    
    df0 = df[df['gap_in_day'] >= next_monday_gap]

    return df0