"Tiptoe Through the Tulips..."
-- Uncle Vernon, singing
The letters had been pushed through every available opening into the house, including a small bathroom windows. Uncle Vernon didn’t react well to this.
Uncle Vernon stayed at home again. After burning all the letters, he got out a hammer and nails and boarded up the cracks around the front and back doors so no one could go out. He hummed “Tiptoe Through the Tulips” as he worked, and jumped at small noises. (PS3)
This is Friday (stated), the 27th of July, 1991. In real life, the 27th of July, 1991, was a Saturday, so it's clear that Rowling wasn't referring to any real calendar at this point.