A Google search of the phrase “dog days of summer” reveals: In ancient times, when the night sky was unobscured by artificial lights and smog, different groups of peoples in different parts of the world drew images in the sky by “connecting the dots” of stars. The images drawn were dependent upon the culture: The Chinese saw different images than the Native Americans, who saw different pictures than the Europeans. These star pictures are now called constellations, and the constellations that are now mapped out in the sky come from our European ancestors.
The brightest of the stars in Canis Major (the big dog) is Sirius, which also happens to be the brightest star in the night sky. In the summer, however, Sirius, the “dog star,” rises and sets with the sun.