E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.)