I’ve always heard of the “dog days” of summer, and I always thought, somewhat logically, that the expression came from the propensity of dogs to rest in the midday heat. But that’s not the case.
According to National Geographic, the saying has nothing to do with dogs. Rather, it refers to Sirius the dog star. To the ancient Greeks and Romans, Sirius appeared to rise in late July, just before the sun.
“If you go back even as far as Homer, The Iliad, it’s referring to Sirius as Orion’s dog rising, and it describes the star as being associated with war and disaster,” said Jay B. Holberg, author of Sirius: Brightest Diamond in the Night Sky and senior research scientist at the University of Arizona Lunar & Planetary Laboratory. “All throughout Greek and Roman literature, you found these things.”
The phrase “dog days” came into English about 500 years ago, and of course, the meaning has changed as later people have thought up their own explanations.
These days aren’t necessarily the hottest, either. The hottest period can vary each year. And the astronomical dog days can come at different times depending on where you are in the world.
Finally, remember the stars are completely independent from our seasonal calendar.
“Our Earth is like a spinning top,” said Bradley Schaefer, professor of physics and astronomy at Louisiana State University. “If you toss it onto a table, after it slows down … the pointing direction of the top will slowly go around in circles.” Similarly to a top, “the Earth’s rotation is kind of wobbling around.”
“The calendar is fixed according to certain events, but the stars have shifted according to the way that the Earth wobbles,” said Larry Ciupik, astronomer at Adler Planetarium and director of the Doane Observatory. “So in about 50-some years, the sky shifts about one degree.”
So the star pattern changes on a 26,000-year cycle. In about 13,000 years, Sirius will be rising with the sun in mid-winter. Then how will we explain the “dog days of winter”?
The quotes and picture come from 
http://news.nationalgeographic.com/2015/07/150710-dog-days-summer-sirius-star-astronomy-weather-language/
The ancient Greeks thought of the constellation Canis Major as a dog chasing Lepus, the hare. The star Sirius is the dog’s nose; the Greeks called it the “dog star.”