When are the 40 days of astronomical "Dog Days"?

 July 3 - August 11 annually

the most sultry days of the summer season

they conclude with the heliacal (at sunrise) rising of the 

Dog Star, Sirius.

For the ancient Egyptians, the star SIRIUS

appeared just before the Nile's flooding,

so they used the star as a "WatchDog" for that flooding.