The reason behind the movie title "Dog Day Afternoon"

Question

So I recently was able to watch the movie Dog Day Afternoon and finally realized what a great movie it is! This movie surely made me a fan of Al Pacino.

Now, normally the title of a movie depicts what the movie is all about, or gives a small gist of the movie, or has some reference to the story in a way or another. I believe a proper title for a movie plays a role in creating some eagerness for those who are unaware of that movie or have only read something about that movie.

However the title of this movie wasn't that clear to me and made me wonder, What is the reason or story behind the title of this movie?

Answer 1

IMDb Trivia page:

What does the title mean?

The phrase dog days refers to the hot, sultry days of summer (July and August in the Northern hemisphere and January and Februrary in the Southern hemisphere). The name comes from the ancient Roman dies caniculares, which was associated with the 'dog star' Sirius (so named because it is the brightest star in the constellation Canis Major [Large Dog]). The Romans sacrificed a brown dog at the beginning of the Dog Days to appease the rage of Sirius, believing that the star was the cause of the hot, sultry weather. In modern times, the term refers to those hot, sleepy afternoons when dogs (and people) prefer to lay around and languish in the summer heat.

Wikipedia:

The title refers to the sultry "dog days" of summer.

Answer 2

I hope this also gives you an new slant on the meaning. A Dog(s) Day in Australian slang and also will fit in with the title is as follows.

"A dog (of a) day " means a variety of things but in summating it means Everything and anything, plans made, emotions, work, friendships, weather etc has turned Bad very Bad unexpectedly and Quickly and whith little or no recourse or chance of making better.

Below is a link to Australian Slang explaining many varied uses and combinations used. Source : Me being Australian and http://www.slang-dictionary.org/australian-slang/Dog