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.