One of my friends, a teacher, sends The Teacher's Corner to me because it shows interesting daily events. She asked, "Why do you think that July 3 would be the first day of the Dog Days of Summer when it's usually hotter in August?
In researching, I learned that the phrase has nothing to do with dogs lolling about in sweltering heat. To the Romans, the "dog days" occurred around the day when Sirius appeared to rise above the sun, and coincidentally, that usually happened in early July.
Translated from Latin--dies caniculares--means "puppy days", thus the term "dog days".