Ever wonder why very hot summer days are called “dog days”?

From July 3 to August 11, the Dog Star Sirius rises and sets with the sun. 

In early Rome, it was believed that the Dog Star was somehow responsible for the extremely hot weather during this period, and for this reason these very hot days were called dies caniculares, or days of the dog.  This is why, today, we call hot summer days, “dog days.”

From: Every Wonder Why? by Douglas B. Smith