The "dog days of summer" is an aphorism that is often used to describe the hottest period of the summer season (in the northern hemisphere, July-early August; in the southern hemisphere, January-early February). In modern times the saying is believed to stem from the fact that dogs are much slower in their movement and actions during the summer, as if the extreme heat is even too much for a dog to handle. As much as this may make sense, the saying has been around since ancient times.
The Greeks are believed to be the first to use the term "dog days," but it was actually the Romans that first associated the humidity of summer to the constellation Canis Major and its brightest star, Sirius.
Canis Major is the Latin name for "Greater Dog," and it shows the image of a larger dog in counterpart with Canis Minor, which shows the image of a smaller dog. Sirius is the brightest star in the night sky and is often called the "dog star," sitting in the sky as the nose of the dog constellation.
Although Canis Major is a winter constellation (meaning that it is visibly seen in the winter sky), Sirius shines brightly year-round, so brightly that the Romans believed it radiated additional heat down to the Earth, causing the increase in temperature and humidity. This was accompanied by the fact that Sirius rose and set at the same time (or roughly the same time) as the Sun, or correspondingly with the heliacal rising, and was in conjunction with the sun in mid-late July.
The Romans labeled this period of summer the "dog days" in corroboration with the fact that the "dog star" was the reason for the excess heat and sultry temperatures. Homer wrote about Sirius and the dog days in The Illiad, giving it the power to bring drought and disease to human kind during its summer wrath, and because of this, the dog days were considered far more dangerous than they are in current times.
The ancient Romans considered the dog days of summer as July 24-August 24, and although they're still considered to be this by many European cultures, the precession of the equinoxes has caused these dates to change to July 3-August 11, as listed in The Old Farmer's Almanac.
It's actually the precession of the equinoxes that causes the dog days of summer, as now known. The hottest days of summer occur because of the tilt of the earth's axis. During the summer season, it's tilted toward the sun, and during these days it's tilted the farthest, therefore causing the earth to be closest to the sun. The tilt of Earth's axis gradually shifts over a period of 26,000 years, and because the rotational tilt occurs earlier in the season now than it did back then, the hottest days now come earlier.
The dog days of summer bring about an abundance of lethargy, but the dog star is not the source of this excess heat. The Romans were creative people, and although their explanations are fun to consider, they're far from scientific. Sirius continues to be the brightest star in the solar system, and it's connection to the dog days of summer will forever be surmised.