def max_change_day(prices):
    """Returns the day (index) of the maximum single day change in price."""
    diff_list = []
    for x in range(len(prices)-1):
        diff_list += [prices[x+1] - prices[x]]
    
    maxindex = 0
    maximum = diff_list[0]
    
    for x in range(len(diff_list)):
        if diff_list[x] > maximum:
            maximum = diff_list[x]
            maxindex = x + 1
    return maxindex