The dog days of summer describe that general time between July and August when it's hot you're feeling lazy as a dog. Many people think that's where the word comes from, but it gets a little more involved than all that. In fact, the dog days occur because of an astronomical reason: they happen when the star Sirius is in the night sky. Only recently was an association with heat made! Dog days as a phrase is a calque on the Latin term dies caniculares, which translate literally into our saying. But why the connection between Sirius rising and dogs? Well, as any Harry Potter fan can tell you (and this is one hundred percent true), Sirius is Latin for "dog", and the star Sirius is often referred to as the "dog star". Thus the correlation. Interestingly enough, the rising of Sirius has shifted over time; during the times of the summer solstice, it peaked around mid-June instead of August. Perhaps that's why the heat link was forged later...
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd