This has to due with sacrifice of the woman. In the text is says, "'It isn't fair, it isn't right,' Mrs. Hutchinson screamed, and then they were upon her." Mrs. Hutchinson is being sacrificed for the towns lottery, and everyone acts like it's normal since they do this every year.
The boys are collecting rocks for the lottery that they have every year. This had foreshadowed the rock throwing at the end of the story.
In the text it says, "The morning of June 27th was clear and sunny, with the fresh warmth of a full-summer day; the flowers were blossoming profusely and the grass was richly green." This really sets the mood of this scene as it gives off a happy and calming vibe to it.