Good question. I hope this is a good answer.
Let's begin with a slightly different phrase, "the dog days of summer." Dog days are not about or connected to the lovable cannines which many of us enjoy. Dog days are actually referring to the star "Sirius", also known as the dog star, found in the constellation of Canis Major, deriving its name from the Greek word '"seirios", which means "scorching".
The star sirius is most visible in our hemisphere during the summer months as the brightest star in the heavens (not Polaris, the northern star).
So, "dog day afternoon" actually means nothing more than a hot summer afternoon.
Thanks for asking. I've been carrying this bit of trivia around in my head for years, and this is the first opportunity I've had to share it.