User blog:Aaronsclee/When is the Hogwarts acceptance letter posted?

Yes, that. It says on the Hogwarts acceptance letter, "We await your owl by no later than July 31." From this, presumably the letter is sent on the future student's birthday, or a set date before the 31st. Which one is it? What is the date if it is the latter? And also do you have to be 11 before the day the letter comes, 31 July or 1 September?