def weekday_difference(weekday_ref, weekday_target):
    """
    Return
      1     if `weekday_target` is one day ahead of `weekday_ref`
      0     if `weekday_target` and `weekday_ref` are the same
      -1    if `weekday_target` is one day behind of `weekday_ref`
      None  otherwise
    """
    if weekday_target == weekday_ref:
        return 0
    if (weekday_target + 7 - weekday_ref) % 7 == 1:
        return 1
    if (weekday_target + 7 - weekday_ref) % 7 == 6:
        return -1
    return None