(USDA Historical Photo Archive)
The 2006 Old Farmer's Almanac says that the Dog Days of summer last from July 3rd to August 11th this year. These are normally the hottest and least healthy days of the year. They are called the Dog Days because of the Dog Star, Sirius and its movements this time of year.
So, keep hydrated when you're out in your patch, and try to accomplish heavy, physical activity in the cool of the morning or early evening.