E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Dog-days.
Days of great heat. The Romans called the six or eight hottest weeks of the summer caniculars dis.
According to their theory, the dog-star or Sirius, rising with the sun, added to its heat, and the dog-days bore the combined heat of the dog-star and the sun. (July 3rd to August 11th.)