The dog days refer to “the sultry part of the summer, supposed to occur during the period that Sirius, the Dog Star, rises at the same time as the sun: now often reckoned from July 3 to August 11”. More generally, this phrase can also refer to “a period marked by lethargy, inactivity, or indolence”.
The Encyclopedia Britannica has an interesting bit of history about the dog days:
"The name originated with the ancient Greeks, Romans, and Egyptians; they believed that Sirius, the dog star, which rises simultaneously with the Sun during this time of the year, added its heat to the Sun's and thereby caused the hot weather. Their belief that dogs were subject to spells of madness at this time also may have contributed to the name."
Here in Los Angeles, the dog days came a little later this year. It’s been hot lately! Anyone else suffering from spells of madness? I know I am.
Bonus word: the adjective canicular means “relating to the dog days” or the Dog Star, since the Latin name for the star comes from canis, the Latin word for “dog”.