def set_days_in_month(month_picked):
    """
    Changes the values in the slider based on how many days are
    in the month selected.

    Args:
        month_picked (str): month June through Sept
    Returns:
        days (int): number of days in that month
        marks (dict): dictionary of day intervals for corresponding tick
            marks on the slider
    """
    if month_picked in ['July', 'August']:
        days = 31
        marks = {1: '1', 10: '10', 20: '20', 31: '31'}
    else:
        days = 30
        marks = {1: '1', 10: '10', 20: '20', 30: '30'}

    return days, marks