Rick Moran led us to the real meaning of Dog Days. Via Wiki:
For the ancient Egyptians, Sirius appeared just before the season of the Nile's flooding, so they used the star as a "watchdog" for that event. Since its rising also coincided with a time of extreme heat, the connection with hot, sultry weather was made for all time.
Sirius is the shiny dog collar tag in Canis Major: