def suffix(day_of_month):
    """
    Returns suffix for day of the month to make
    dates look pretty for eventual plotting
    
    """
    if day_of_month in [1,21,31]:
        ending = 'st'
    elif day_of_month in [2,22]:
        ending = 'nd'
    elif day_of_month in [3,23]:
        ending = 'rd'
    else:
        ending = 'th'
    return ending