"I WANT MY LETTER!"
— Harry Potter
Harry receives his first letter from Hogwarts, which Uncle Vernon burns. After he boards up the letter box, letters begin to appear rolled up inside eggs and down the chimney.
The dates are determined by counting back from the established date of July 31 for Harry's birthday.
We are told the days on which these dates fall:
Monday. This reminded Harry of something. If it was Monday -- and you could usually count on Dudley to know the days the week, because of television -- then tomorrow, Tuesday, was [Harry]'s eleventh birthday. (PS3)
In the "real" 1991, however, July 31 fell on a Wednesday.
This event appears on Pottermore as chapter 3, moment 1 of Harry Potter and the Philosopher's Stone.