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1530s, from Latin dies caniculares, from Greek; so called because they occur around the time of the heliacal rising of Sirius, the Dog Star (kyon seirios). Noted as the hottest and most unwholesome time of the year; usually July 3 to Aug. 11, but variously calculated, depending on latitude and on whether the greater Dog-star (Sirius) or the lesser one (Procyon) is reckoned.
The heliacal rising of Sirius has shifted down the calendar with the precession of the equinoxes; in ancient Egypt c.3000 B.C.E. it coincided with the summer solstice, which also was the new year and the beginning of the inundation of the Nile. The "dog" association apparently began here (the star's hieroglyph was a dog), but the reasons for it are obscure.
The hot, muggy days of summer. The Romans associated such weather with the influence of Sirius, the dog star, which is high in the sky during summer days.
periods of exceptionally hot and humid weather that often occur in July, August, and early September in the northern temperate latitudes. The name originated with the ancient Greeks, Romans, and Egyptians; they believed that Sirius, the dog star, which rises simultaneously with the Sun during this time of the year, added its heat to the Sun's and thereby caused the hot weather. Their belief that dogs were subject to spells of madness at this time also may have contributed to the name. Because people tended to become listless during the dog days, Sirius was held to have a detrimental effect on human activities