Summer is the best time of year. Avoiding Dudley and his gang is easy if he can finish his chores before Dudley wakes up at noon, since they all avoid the public library the way demons would avoid churchyards if demons existed. The only hard part is getting home when it's time to cook dinner, since if they spot him on open ground they're nearly always faster. Also this summer Dudley has a stick, because apparently the school he's going to this fall requires them. Bruce is going to a different school for once, with a worse reputation but hopefully less melee combat and definitely no Dudley.

One bright sunny morning, the mail comes, as usual. Vernon tells Dudley to get it, Dudley whines for him to make Bruce do it, and Vernon obliges. 

Bruce isn't sure whether to give Uncle Vernon credit for trying to get Dudley to do something, or think he's stupid for not predicting the inevitable whining and just having Bruce get it straight off. He ponders this mystery while going and getting the mail.

Junk, junk, bill, postcard, someone wrote him a letter what the hell, bill. The letter with his name on it gets hastily folded in quarters and shoved down his too-large sock, and he brings the rest back to Uncle Vernon while trying to look even more ignorable than usual.

Uncle Vernon collects the rest of the mail from Bruce and, conveniently, ignores him. 

Being ignored: it's pretty great. Bruce does the washing-up and then takes the first opportunity to shut himself in his cupboard and check his mail. He has to hold the letter up to the crack between the door and the wall to make out the words, but it does still say "Bruce Potter, the cupboard under the stairs, Number Four, Privet Drive, Little Whinging, Surrey". In green ink, for some reason, and on oddly thick paper.

It turns out to contain the following:

Hogwarts School of Witchcraft and Wizardry

Headmaster: Albus Dumbledore (Order of Merlin, First Class, Grand Sorc., Chf. Warlock, Supreme Mugwump, International Confed. of Wizards)

Dear Bruce Potter,

We are pleased to inform you that you have been accepted at Hogwarts School of Witchcraft and Wizardry. Please find enclosed a list of all necessary books and equipment. Term begins on September 1. We await your owl by no later than July 31.

Yours sincerely,

Minerva McGonagall Deputy Headmistress

What on Earth? Okay, first of all, magic isn't real, so this is clearly an attempt to mess with him. Major points for creativity, at least relative to Dudley's attempts to mess with him, but that just raises the question of who did it and why. The key is probably that bit with "we await your owl". He doesn't have an owl and has no way of providing one to whoever sent the letter even if he did. Presumably they know that (they know where he sleeps) and are looking for an excuse to punish him for failing. Possibly they just want him to believe there really is a magic school and feel bad about missing out on it, but this letter seems like a lot of effort for someone to go to just for that.

Honestly, it seems like too much effort for anyone to put into messing with Bruce, full stop. Maybe whoever sent it is getting something else out of this too. His aunt and uncle are definitely the main suspects; nobody outside the house knows where he sleeps unless he mentioned it to Mrs. Figg at some point. In which case, they'll already know he has it and be expecting some sort of reaction. Maybe if he doesn't react, they'll think he might not have read it and do something revealing.

Bruce reads the enclosed book list, concludes that this is definitely some sort of made-up prank, and stuffs the letter under his mattress.

The next day, the only mail is another, identical letter. 

Well. That suggests his nonresponse to the first one was unexpected or unsatisfactory; on the other hand it means the sender doesn't give up easily. And it's got to be his relatives because postal turnaround time is such that nobody else could have known to send another one fast enough. Under the mattress with it.

The nest day there are six letters.

Whaaaat.

What happens if he just. Picks them up and delivers them to Uncle Vernon as if he hadn't noticed his name on them? Maybe he'll do something informative before or after the inevitable something unpleasant.

Vernon glances at them, turns away to sip his coffee, then sputters and double-takes and starts turning purple as he stares at the letters. He violently tears all six of them to shreds and then returns to his breakfast, pretending unconvincingly that nothing happened. 

Woah. Okay, pretending that didn't happen, yes. So clearly Uncle Vernon either didn't send them or is doing a really good impression of not wanting him to read them. He belatedly realizes he should have kept one of the six to see if it was exactly the same as the first.

If Uncle Vernon didn't do it, there's still a slight chance Aunt Petunia did, but it seems very weird for them to be at odds over something like this, and the former clearly knows something he doesn't. Maybe if he listens at doors for the rest of the day Uncle Vernon will say something about the letters to Aunt Petunia.