As I understand it, it's been more or less "accepted" that September 1st is the date that determines which year of Hogwarts you're in. But I also do not believe this has been explicitly confirmed anywhere.
Looking only at things JK Rowling has written or said, what do we know about the cut-off date? Has it been said anywhere? If not, what are the closest bounds we can establish based on known character birthdays? (If using birthdays be careful to demonstrate that the year of the birthday is itself known, and not just derived from the assumption that Sept 1st is the cut-off.)