Why are the hottest days of the year called “dog days”?

The hottest days in Europe mostly occur in the month of July. According to an old superstition, this was also the period when dogs had a tendency to go mad. The expression “dog days” is a translation of the Latin “caniculares dies” and it has been around since the times of the ancient Romans. But contrary to popular belief, “dog days” has nothing to do with dogs on earth; it has more to do with the dog in the sky! In this case, it refers to Sirius, the Dog Star. During the height of summer, the Dog Star, Sirius, rises along with the Sun. The ancient Romans believed that this star added to the heat of the Sun, and in the process caused the weather to become unbearably hot.
 Source:  ‘Know Your English’ Series – The Hindu Daily, August 22 , 2005.