The idea of "dog days' originates from the constellation, Canis Major. In Latin, Canis Major translates to "greater dog" and it contains Sirius, most notible as the brightest star in the night sky and also known as the "dog star."
In ancient Europe, Canis Major was associated with dogs and Sirius, also known as the "scorching star," since the summer heat occurs just after Sirius is first spotted on the horizon for the new year signified the beginning of summer. The ancient Greeks referred to this time in the summer as dog days, as only dogs would be adventure out in the heat, leading to the star being known as the Dog Star. Consequently, the constellation was named after it.