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You've likely heard the phrase. And, it's not usually uttered with fondness.
The "Dog Days of Summer" represents the hottest, most sweltering part of the summer. In the northern hemisphere, this is July and August.
The phrase seems to have originated with the Greeks. It's referred to in the "Iliad."
The Romans blamed the heat on the position of the star "Sirius". During July and August, Sirius is very close to the sun, as shown in the graphic below.
Sirius is also known as the "Dog Star."
Thus, when the heat arrived in July and August, when the "Dog Star" was close to the sun, the Romans called it the "Dog Days of Summer." Of course, they said it in Latin.
According to Brady's Clavis Calendaria (1813), Dog Days were believed to be a wretched, evil time. "...the Sea boiled, the Wine turned sour, Dogs grew mad, and all other creatures became languid; causing to man, among other diseases, burning fevers, hysterics, and phrensies."