Where does the term “dog days of summer” come from?

The “dog days of summer” refer to the hot and humid days between July and August. It comes from the Dog Star, Sirius, which is part of the constellation Canis Major. Each summer between July and August the constellation appears in the northern hemisphere during what is usually the hottest time of the summer. It is the brightest star in Canis Major and is easily recognizable with the naked eye.
 

Source: http://voices.yahoo.com/dog-days-summer-meaning-phrase-3791650.html