In modern times, the dog days of summer in the Northern Hemisphere generally refer to July and August – the hottest, muggiest, and most miserable days of the year.
However, the expression has its origins in ancient Greece and Rome as an allusion to the star Sirius, also known as “the dog Star”. Sometime in July, Sirius aligns with the sun, and since Sirius is the second brightest star in our sky, the Greeks and Romans thought it literally added warmth to the sun, making the days that much muggier. They viewed the dog days as perilous times of drought, unrest, and bad luck, all of which could lead to madness. However, this connotation is somewhat dramatic and has mostly been dropped.