Meaning: The hottest, most sultry days of summer..
Origins: This saying has nothing to do with dogs. The expression originated in Roman times as 'canicularius dies,' 'days of the dog,' and was an astronomical expression referring to the dog star Sirius, or possibly Procyon. The Romans linked the rising of the Dog Star, the most brilliant star in the constellation, 'Canis Major,' with the sultry summer heat, believing that the star added to the extreme heat of the sun. 'Canicular days,' of course, have nothing to do with heat from the Dog Star, but the ancient expression remains popular after more than 20 centuries."
From "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997, http://www.phrases.org.uk/bulletin_board/10/messages/722.html