According to Kara Kovalchik of Mental Floss, “The official ‘dog days’ of summer begin on July 3 and end on August 11. So how did this time frame earn its canine nickname? It turns out the phrase has nothing to do with the poor pooches who are forever seeking shade in the July heat, and everything to do with the nighttime sky.
Sirius, the Dog Star, is the brightest star in the sky. The ancient Greeks noticed that in the summer months, Sirius rose and set with the Sun, and they theorized that it was the bright, glowing Dog Star that was adding extra heat to the Earth in July and August.”