birthday.py

from datetime import datetime

number_endings = {
    1: 'st',
    2: 'nd',
    3: 'rd'
}

today = datetime.now()
todays_day = today.day

# get the right ending, e.g. 1 => 1st, 2 => 2nd
# but beware! 11 => 11th, 21 => 21st, 31 => 31st

# test your code by forcing todays_day to be something different
# todays_day = 11
# todays_day = 2

ENDING = 'th'

def get_ending(day):
    day = int(day)
    if day < 10 or day > 20:
        # x % y (mod) will give the remainder when x is divided by y
        number = day % 10
        # look up number in number_endings
        # and if you find it as a key, put the value in ending
        if number in number_endings:
            ending = number_endings[number]
        else:
            ending = ENDING
    else:
        ending = ENDING
    return ending

# make this print ending, not 'th'
print "Today is the {}{}".format(todays_day, get_ending(todays_day))

birthday = raw_input("What day of the month is your birthday? ")

# make this print the birthday, and the right ending
print "Your birthday is on the {}{}!".format(birthday, get_ending(birthday))