The dog days of summer seems a natural expression. In hot weather dogs pant and lie as flat on the ground as they can to avoid the heat. The expression doesn't originate with the behavior of dogs during the summer, however. It reaches farther back in time to some mistaken astronomical impressions of the Greeks and Romans.
During hot weather, particularly the period from mid-July through August, the star Sirius, part of the constellation Canis Major, rises and sets with the sun. It was believed by the ancients that this conjoined rising and setting produced extra heat for the Earth. This explanation was used to account for the hot and sultry heat produced during the summer months.
Of course, we now know that, while Sirius produces a tiny amount of light energy in our direction, it isn't responsible for the dog days of summer. The hot weather is actually caused by the tilt of the Earth in relationship to the Sun. In the Northern Hemisphere, this slight tilt allows the Earth to receive more direct sunlight beginning on June 21st. In the Southern Hemisphere, the dog days would actually occur during December, when countries like Australia are tilted slightly more toward the sun and receiving a greater amount of sunlight.
Another mistaken impression about the dog days of summer is that they occur when the Sun is closest to the Earth. This is actually not true, though it is the case that the Earth's distance from the Sun varies slightly during its orbit. Tilt, instead of distance, determines the amount of direct sunlight the Earth gets, and during the Northern Hemisphere in July, the planet is actually farthest from the Sun.
The days described as the dog days of summer are often given a specific date, though there has been different opinions on the actual dates. Some describe the date as between 3 July and 11 August, and others suggest 6 July through 17 August. Most agree that the dog days last 40 days.
The phrase picked up new meaning in the 19th century. It was believed that dogs would most often contract rabies during this time period. A scene in To Kill A Mockingbird suggests this belief continued well into the 20th century, when a mad dog is shot by Atticus Finch. All notice that it isn't the time of year for rabies to occur since the shooting occurs well before the summer months.