"'Scuse me, but is one of you Mr. H. Potter? Only I got about an 'undred of these at the front desk."
-- Owner of the Railview Hotel, Cokeworth (PS3)
While eating breakfast — stale cornflakes and cold tinned tomatoes on toast — the owner of the hotel lets them know that about a hundred letters have arrived at the hotel for Harry. The address on each one says “Mr. H. Potter, Room 17, Railview Hotel, Cokeworth.”
Uncle Vernon sees the letters and realizes that someone is watching them and knows where they are, right down to which room Harry stayed in.
Timeline Notes
Today is the day before Harry's eleventh birthday, which would be the 30th of July. According to the book, it's a Monday.
"It's Monday," [Dudley] told his mother. "The Great Humberto's on tonight. I want to stay somewhere with a television. "
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)