The phrase dates back to ancient Rome, thousands of years ago. “Caniculares dies,” or days of the dogs, was what the Romans called the period from the first week of July to the second week of August. Therefore, the dog days of summer only refer to the last part of the summer, not the whole season.
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.