Creating ( In Python ) a fully working google apps engine Birthday engine. Anyone that needed help on Udacity can look here.

Birthday Plugin

import webapp2
import cgi

form="""
<form method="post">
    What is your birthday?
    <br>
    <label>Month <input type="text" name="month" value="%(month)s">
    </label>
    
    <label>Day <input type="text" name="day" value="%(day)s">
    </label>
    
    <label>Year <input type="text" name="year" value="%(year)s">
    </label>
    
    <div style="color: red">%(error)s</div>
    <input type="submit">
</form>
"""

class MainPage(webapp2.RequestHandler):
    def write_form(self, error="", month="", day="", year=""):
        self.response.out.write(form % {"error": error,
                                        "month": escape_html(month),
                                        "day": escape_html(day),
                                        "year": escape_html(year)})
    def get(self):
        self.write_form()
    def post(self):
        user_month = valid_month(self.request.get('month'))
        user_day = valid_day(self.request.get('day'))
        user_year = valid_year(self.request.get('year'))
        if not (user_month and user_day and user_year):
            self.write_form("Not Valid",user_month, user_day, user_year)
        else:
            self.redirect("/thanks")
class ThanksHandler(webapp2.RequestHandler):
    def get(self):
        self.response.out.write("Thanks! That's a totally valid date! cough not day. DATE")
months = ['January',
          'February',
          'March',
          'April',
          'May',
          'June',
          'July',
          'August',
          'September',
          'October',
          'November',
          'December']
month_abbvs = dict((m[:3].lower(),m) for m in months)
def valid_month(month):
    if month:
        #cap_month = month.capitalize()
        #if cap_month in months:
        #    return cap_month
        short_month = month[:3].lower()
        return month_abbvs.get(short_month)
def valid_day(day):
    if day and day.isdigit():
        day = int(day)
        if day > 0 and day <= 31:
            return day
def valid_year(year):
    if year and year.isdigit():
        year = int(year)
        if year > 1900 and year <2020:
            return year
def escape_html(s):
    return cgi.escape(s, quote = True)
app = webapp2.WSGIApplication([('/',MainPage), ('/thanks', ThanksHandler)], debug=True)

This code will crash on submission of invalid date. It is because you are passing the boolean values that you are using to check if dates are valid or not. I have made a fork of your code and made updates, you can update the code.