Dog Days

Like us, Orion was a dog lover. He had two loyal canine companions, the big dog and the little dog, who followed him, even in death.

Presumably he was stuck up in the sky so that Artemis (Zeus's daughter, who loved him) would stop grieving over his death, since she would still be able to see him at night  The two dogs were sent to keep him company.

Since Sirius, the brightest star in the sky (two stars actually--It's a binary system) is in the Canis Major, or big dog constellation, it is called the dog star.
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In Roman times (but not now) Sirius appeared in the sky  just before sunrise just as summer was beginning. The Romans referred to the days when Sirius could be seen as "dies caniculares," "dog days" to those of us who don't speak Latin. When the dog days arrived, so did the heat. To make matters worse, werewolves came out at the same time, as well as many other evils. Sirius was blamed for them all. In order to appease him, the Romans sacrificed a brown dog.

Their sacrifice appears to have been halfway successful . Although summers are still hot, the werewolves, at least, are gone.
http://www.business-esolutions.com/starmyths/myths/orion.htm
http://www.astro.wisc.edu/~dolan/constellations/constellations/Canis_Major.html
http://miprinting.blogspot.com/2011/06/mi-printing-word-mythsdog-days.html
This is the only reference I could find that links Sirius specifically with werewolves.