The Romans referred to the dog days as diēs caniculārēs 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 (see, e.g., Aristotle's Physics, 199a2).
The Dog Days originally were the days when Sirius 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.