def get_next_active_day(days, current_day, active_days):
    """Gets the next active day, often this will simply be the next day, i.e. if you set the active days as Mon - Fri
    and today is Mon, then the next active day is Tue. However is today is Fri then the next active day will be Mon.

    If say our active days are Mon - Fri and today is say ``Fri``, then next active day is ``Mon``. So instead of adding
    one to the index and getting ``Sat`` we will simple go to the start of our list index 0 which is ``Mon``. However
    most of the time the next day will be the active day i.e. today is ``Mon`` so we will simply just add one to our index
    to get ``Tue``.

    Args:
        days (list): Of days Mon - Sun.
        current_day (str): The current day i.e. ``Monday``.
        active_days (list): Of days we want the live light to be on.

    Returns:
        str: Next day the live light should be switched on.

    """
    common_days = [day for day in days if day in active_days]
    current_day_index = common_days.index(current_day)
    days_in_common = len(common_days)
    next_active_day = (
        0 if current_day_index + 1 == days_in_common else current_day_index + 1
    )
    return days[next_active_day]