Dog Days

There technically is such a thing as the "dog days of summer," but it has nothing to do with actual dogs. From the Columbia Encyclopedia via Fact Monster:

Dog Days is the name for the most sultry period of summer, from about July 3 to Aug. 11. Named in early times by observers in countries bordering the Mediterranean, the period was reckoned as extending from 20 days before to 20 days after the conjunction of Sirius (the dog star) and the sun.

In the latitude of the Mediterranean region this period coincided with hot days that were plagued with disease and discomfort.

The time of conjunction varies with difference in latitude, and because of the precession of the equinoxes it changes gradually over long periods in all latitudes.