The Old Farmer’s Almanac lists July 3, incorrectly according to some, as the onset of Dog Days. The Romans referred to the Dog Days as diēs caniculārēs and associated the hot weather with the star Sirius, the brightest star in the constellation Canis Major (Large Dog).
The Dog Days originally were the days when Sirius rose just before or at the same time as sunrise (heliacal rising), which actually takes place the first part of August. 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.
Others believe the phrase is in reference to the conspicuous laziness of domesticated dogs during the hottest days of the summer.