The earliest reference to this expression goes back to the Ancient Egyptians. They noted that the rising of the star Sirius began the hottest part of the summer. The star’s hieroglyph is a dog. Sirius would appear in Egypt, after about a 70 day absence, just before the season where the Nile typically floods, so it is thought the star’s hieroglyphic symbol 'watchdog'.
Romans and Greeks also referred to dog days and would often make
sacrifices to Sirius, including sacrificing dogs to appease Sirius
with the hope it would result in a mild summer and protect crops
Sirius is the brightest star in the Canis Major (Latin for “Greater