Curley's wife meets her death on a Sunday afternoon. This is indicated by the fact that all the men, except Lennie, are pitching horseshoes outside the barn. Lennie has the day off too, and he is inside the barn playing with his puppy. He would probably be incompetent at pitching horseshoes and not invited to participate. Crooks is not allowed to sleep in the bunkhouse, but he can pitch horseshoes with the white men. Apparently he is very good at it, too. Earlier Carlson had said, "Jesus, how that nigger can pitch shoes." This statement was merely intended to indicate that Crooks would be pitching horseshoes with all the other men and therefore would not be in his little room adjacent to the barn. Lennie and Curley's young wife would be all alone. The audience would sense impending trouble.
From outside came the clang of horseshoes on the iron stake, and then a little chorus of cries.
Steinbeck called his book "a playable novel." It was written in such a way that it could be speedily and easily converted into a stage play. Steinbeck already had an agreement to provide a script for a play to open in New York the same year the book was published. The horseshoe-pitching is only represented by sounds, which could easily be produced offstage in the play by a prop man banging iron against iron while a few stagehands occasionally emitted shouts and cheers. In the book Steinbeck simply states:
It was Sunday afternoon.
But this would have to be shown in a stage play. The audience would understand it was Sunday because the men were not working.
Curley's wife comes into the barn. She is probably hoping to run into Slim. She thinks he might be there because he must spend some time looking after his dog and her new pups. They are not having an affair--although Curley suspects the worst. Slim is the only man who is kind to her and talks to her. She runs into Lennie instead and stays to talk to him in lieu of anything better to do. The purpose of all the business about horseshoes is to show (1) that it is Sunday, (2) that all the men including Crooks are pitching horseshoes, and (3) that Lennie and Curley's wife are all alone in the barn.
The death of Curley's wife has to take place on a Sunday. Otherwise Lennie and all the other men would be out working in the fields. Although the men are free for the day, there is nothing for them to do but pitch horseshoes, which costs nothing.