The phrase Dog Days of Summer describes the most oppressive period of summer, between July 3rd and August 11th each year. But where did the term come from? And what does it have to do with dogs?
The Astronomy Connection. Many people believe the phrase stems from the fact that dogs tend to be sluggish during the hottest days of summer (aren’t we all?), while others still say the days are so hot it causes dogs to go mad. But the name is actually a reference to the fact that, during this time, the Sun occupies the same region of the sky as Sirius, the brightest star visible from any part of Earth and part of the constellation Canis Major, the Greater Dog. This is why Sirius is sometimes called the Dog Star.
In the summer, Sirius rises and sets with the Sun. On July 23rd, specifically, it is in conjunction with the Sun, and because the star is so bright, the ancient Romans believed it actually gave off heat and added to the Sun’s warmth, accounting for the long stretch of sultry weather. They referred to this time as diēs caniculārēs, or “dog days.”
Thus, the term Dog Days of Summer came to mean the 20 days before and 20 days after this alignment of Sirius with the Sun—July 3 to Aug. 11.