求助 新学python 不知错那了？

#! /usr/bin/env python
# list.sort()
months=[
        'January=1'
        'February=2'
        'March=3'
        'April=4'
        'May=5'
        'June=6'
        'July=7'
        'August=8'
        'September=9'
        'October=10'
        'November=11'
        'December=12'
]
endings=['st','nd','rd'] + 17*['th'] +['st','nd','rd'] +7 *['th']+['st']
#取得输入值
year=raw_input('year:')
month=raw_input('month:')
day=raw_input('day:')
 
#取得的值
month_number= int(month)
day_number=int(day)
 
 
month_name =months[month_number-1]
ordinal=day+endings[day_number-1]
 
 
print month_name +' '+ordinal+','+year

运行 总是提示：File "./news", line 28, in ?
    month_name =months[month_number-1]
IndexError: list index out of range     请大侠给与指正 到底是哪错了？！