From Wikipedia
In Ancient Rome, the Dog Days extended from July 24 through August 24 (or, alternatively July 23-August 23). In many European cultures (German, French, Italian) this is still the period to be the time of the Dog Days.
The Old Farmer’s Almanac lists the traditional timing of the Dog Days as the 40 days beginning July 3 and ending August 11, coinciding with the ancient heliacal (at sunrise) rising of the Dog Star, Sirius. These are the days of the year when rainfall is at its lowest levels.
According to The Book of Common Prayer (1552), the “Dog Daies” begin on July 6 and end on August 17.
In the southern hemisphere dog days do not apply because the Dog Star-Sirius is not easily seen.
The ancient Egyptians noticed the rising of Sirius coincided with the flooding of the Nile and used Sirius as the ‘watchdog’ for this flooding.
Another interesting tidbit, the feast day of Saint Roch, the patron saint of dogs, is August 16. (Which depending upon which time frame you use, falls within the Dog Days of Summer.)
