The phrase is a reference to Sirius, the Dog Star. During the “Dog Days” period, the Sun occupies the same region of the sky as Sirius, the brightest star visible from any part of Earth. Sirius is part of the constellation Canis Major, the Greater Dog.
In the summer, Sirius rises and sets with the Sun. On July 23 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.”
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 August 11 each year. The dates have drifted slightly over centuries because of axial precession (the slow wobble of Earth’s axis), so the literal Sirius conjunction now lands a few days later than it did in Roman times.
