def users_with_birthday(self, month, day):
        """Return a list of user objects who have a birthday on a given date."""

        users = User.objects.filter(properties___birthday__month=month, properties___birthday__day=day)
        results = []
        for user in users:
            # TODO: permissions system
            results.append(user)

        return results
Return a list of user objects who have a birthday on a given date.