Returns day's correct postfix (2nd, 3rd, 61st, etc).

def day_postfix(day):
    """Returns day's correct postfix (2nd, 3rd, 61st, etc)."""

    if day != 11 and day % 10 == 1:
        postfix = "st"
    elif day != 12 and day % 10 == 2:
        postfix = "nd"
    elif day != 13 and day % 10 == 3:
        postfix = "rd"
    else:
        postfix = "th"

    return postfix