By many accounts, today begins the Dog Days of summer, when farmers' almanacs and folk wisdom collectively proclaimed that the hottest, haziest, most humid days of summer would beset the Northern Hemisphere. So where, exactly, did the name Dog Days come from? Outer space, of course.
The Dog Days are named for the Dog Star, Sirius, which is the brightest star (discounting our own sun) visible in the sky. During mid-summer, Sirius is in conjunction with the sun (the two rise and set together), an astronomical coincidence that the ancient Romans and several other cultures couldn't help but notice.
When the two brightest stars sync up and the world simultaneously gets hotter, you tend to blame the Dog Star for the heat. Sirius is in the constellation Canis Major (which the Romans named), so the Romans called this period the carniculares dies — or days of the dogs. Thus, the 20 days before and after the conjunction of the Dog Star and the sun are called Dog Days.
Incidentally, there's no prevailing date for the Dog Days because the date of the Sirius/sun conjunction varies by lattitude. In general, the 2007 Dog Days are July 3 to August 11. Thus, I bid you happy Dog Days, fellow Northern Hemisphereans. To you antipodeans, I merely suggest you thank your lucky stars you aren't getting baked by excess heat from the Dog Star. Siriusly.