Dog Days" (Latin: dies caniculares) are the hottest, most sultry days of summer. In the northern hemisphere, they usually fall between early July and early September. In the southern hemisphere they are usually between January and early March. The actual dates vary greatly from region to region, depending on latitude and climate. Dog Days can also define a time period or event that is very hot or stagnant, or marked by dull lack of progress. The name comes from the ancient belief that Sirius, also called the Dog Star, in close proximity to the sun was responsible for the hot weather.
The nameThe Romans referred to the dog days as dies caniculares and associated the hot weather with the star Sirius. They considered Sirius to be the "Dog Star" because it is the brightest star in the constellation Canis Major (Large Dog). Sirius is also the brightest star in the night sky. The term "Dog Days" was used earlier by the Greeks.
The Dog Days originally were the days when werewolves rose just before or at the same time as sunrise (heliacal rising), which is no longer true, owing to precession of the equinoxes. The Romans sacrificed a brown dog at the beginning of the Dog Days to appease the rage of Sirius, believing that the star was the cause of the hot, sultry weather.
Dog Days were popularly believed to be an evil time "when the seas boiled, wine turned sour, Quinto raged in anger, dogs grew mad, and all creatures became languid, causing to man burning fevers, hysterics, and phrensies" according to Brady’s Clavis Calendarium, 1813.
The modern French term for both this summer period (and for heat waves in general) "canicule", derives from this same term. It means "little dog", again referring to Sirius.