"I'm not sure that'll work, Vernon."
-- Aunt Petunia
Harry gets up at 6am and creeps downstairs. His plan was to wait for the mail carrier on the corner of Privet Drive to intercept the mail for Number Four. However, Uncle Vernon had positioned himself in front of the door in a sleeping bag to prevent just that sort of thing and the plan failed.
Uncle Vernon then nailed up the mail slot to prevent any more mail from arriving.
“See,” he explained to Aunt Petunia through a mouthful of nails, “if they can’t deliver them they’ll just give up.”
“I’m not sure that’ll work, Vernon.”
“Oh, these people’s minds work in strange ways, Petunia, they’re not like you and me,” said Uncle Vernon, trying to knock in a nail with the piece of fruitcake Aunt Petunia had just brought him. (PS3)
We know that this event happens on a Thursday because it specifically said that the events of the next day are on a Friday. Counting the days described in the story from back from the 31st, Harry's birthday, places this letter arriving on the 26th of July.
Interestingly, Harry sets his alarm for 6 am and goes downstairs. One can't help but wonder if the "repaired alarm clock" wasn't working quite right, since the amount of time between that alarm going off and the arrival of the mail seems way too short. Perhaps the alarm clock actually rang at 7am instead of 6, which makes more sense when you consider that the earliest that the Royal Mail delivers is 7am. (https://twitter.com/RoyalMail/status/824246549815431168)