Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Caniculāres Dies

Certain days in the summer, preceding and ensuing the heliacal rising of Canicula, or the dog-star, in the morning. The ancients believed that this star, rising with the sun, and joining his influence to the fire of that luminary, was the cause of the extraordinary heat which usually prevailed in that season; and accordingly they gave the name of dog-days to about six or eight weeks of the hottest part of summer. This idea originated with the Egyptians, and was borrowed from them by the Greeks. The Romans sacrificed a brown dog every year to Canicula, at its rising, to appease its rage. See Sirius.