The dog days refers to the hottest, muggiest, and often most uncomfortable part of summer.
The expression comes from Sirius, the brightest star in the night sky. Sirius is part of the constellation Canis Major, the Greater Dog, so it became known as the Dog Star. In ancient Rome, Sirius appeared to rise with the sun during late summer. Romans connected that period with intense heat, dry weather, and even sickness, so the season became associated with the Dog Star.
Today, people use the phrase simply to mean the peak heat of summer. Most speakers are not thinking about astronomy when they say it.
- We try to finish outdoor work before the dog days arrive.
- Electric bills often rise during the dog days of July and August.
So the modern meaning is straightforward: when someone mentions the dog days, they usually mean the hottest stretch of the year.