What is the "canicula"?

The Latin word canicula, meaning "small dog," is the diminutive form of "canis," source of the English word canine. "Canicula" is also the Latin name for Sirius, the star that represents the hound of Orion in the constellation named for that hunter from Roman and Greek mythology. This is supposed to be visible during a specific date in the summer. Tha's why they call them the "dog days". But, what do you know about the "canicula"? When does it appear? Where? How long does it last? What are the most affected areas? Is it worlwide?