If anyone is a fan of The GodFather, they probably know this prhase.  People use it in various forms and can be used to express the need to 'ready' up for something that might be tough coming down the road.

In The GodFather, Sonny and Clemenza said these lines.

Clemenza: That Sonny's runnin' wild. He's thinking of going to the mattresses already.
Sonny: No, no, no! No more! Not this time, consiglieri. No more meetings, no more discussions, no more Sollozzo tricks. You give 'em one message: I want Sollozzo. If not, it's all-out war: we go to the mattresses

In, 'You've Got Mail', Tom hanks makes reference to the phrase to Meg Ryan about her 'preparing' for war agasinst basically him to fight for her Store.

So if you are gearing up for battle, preparing to take on the world, or the day at hand, or simply a task that is going to be difficult, you could used this phrase.  A friend used this on facebook the other day as a status update. And he simply said it was meant in general towards his day. 

How this came about? Basically, the mafia, would be gearing up for war, and hiring people to fight for them. They would house them in warehouses, or something similiar.  it would be necessary to get mattresses for these soilders to sleep on.  It's the preperation/preparedness of getting ready for Gang/Stree War.