In modern times, the dog days of summer refer generally to July and August in the Northern Hemisphere—the hottest, muggiest, most miserable days of the year.

However, the phrase has its origins in ancient Greece and Rome as a reference 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 was literally adding heat to that of the sun, making the days that much more sweltering. They considered the dog days to be dangerous times of drought, unrest, and bad luck, all of which could all lead to madness. That connotation is a bit dramatic, though, and it’s mostly been dropped.