The constellation Canis Major contains a star called Sirius – also known as the "Dog Star" – and it's only visible in the Northern Hemisphere sky for 40 days each year between July 3 and Aug. 11, or the middle of summer.
It turns out that the ancient Egyptians thought unusually hot weather was directly related to the appearance of the Dog Star.
Most people consider the entire month of August to be the dog days of summer since hot weather can be persistent through the month's end, though by the technical ancient Egyptian interpretation, the dog days are actually over by mid-August.
We now know this isn't true, but the ancient Egyptians believed the sun's energy combined with the energy from Sirius and resulted in excessively hot temperatures.
The Dog Star is the brightest star in the sky (other than our own sun), though Sirius is 546,000 times farther from the Earth than the Sun, so the amount of radiation we receive from it has no effect on the planet's temperatures.
This means the Dog Star actually has nothing to do with hot temperatures in July or August. It's simply an ancient phrase from the Egyptians that has survived for thousands of years that's used to describe uncomfortably hot weather.