As with so much, we must credit the Ancients with the Dog Days of Summer. The Greeks derived dog days, from the Dog Star, Sirius, the most brilliant star in the night sky. This became to the Romans, always ready to adopt Greek culture, caniculares dies: days of the dogs.
In the Northern Hemisphere these is the hot, humid days of midsummer--roughly mid-July to the third week of August--this year 12 July to 20 August. The Greeks believed that a combination of Sirius' light with that of the Sun's effected plants, animals, women and men.