Dictionary.net

The hot, sultry season in July and August. when the dogstar (Sirius) rises with the sun.

dog'd[=a]z, n.pl. the period when the dogstar rises and sets with the sun (generally reckoned July 3d to August 11th)--erroneously supposed to be the time when dogs are specially liable to hydrophobia.

Jours Caniculaires. During these days, comprised between the 24th of July and the 23d of August, the temperature of the air in Europe is generally high and oppressive. They have received this name from the dogstar, Canicula, Sirius- a very brilliant star in the constellation of the great dog, which rises and sets, at this time, with the sun. It was formerly believed to be a period particularly unpropitious to health; that it was dangerous to purge during it; and other phantasies were indulged in regard to it.

The days in which the dogstar rises and sets with the sun.