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
