Brewer's Dictionary of Phrase and Fable/Dog-days
Dog-days. Days of great heat. The Romans called the six or eight hottest weeks of the summer canicula′rēs diēs. 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.)