The expression originated in Roman times as canicularis dies, "days of the dog", and was an astronomical expression referring to the dog Sirius, or possibly Procyon. The Romans linked the rising of the Dog Star, the most brilliant star in the constellation, Canis Major, with the sultry summer heat, believing that the star added to the extreme heat of the sun. "Canicular days," of course, have nothing to do with heat from the Dog Star, but the ancient expression remains popular after more than 20 centuries.
My sister and I, August 1973...the day I realized I hated watermelon. It's a sticky mess on a hot summer day...now I prefer seedless watermelon cut into cubes served indoors with the AC on full blast.