The first act of any story concludes at the point of no return. We see a lot of anticipation in "The Lottery" as the villagers gather in the square and their children gather stones in a vast pile. But the proceedings don't really start until, in the fourth paragraph, Mr. Summers emerges with the black box, which he settles on Mr. Graves's three-legged stool.
Once the box, which is the physical manifestation of this village's traditions, enters the public square, everyone in the square has no choice but to participate in the lottery. There's no going back.
The second act takes us from the point of no return (basically, the beginning of the lottery) to the moment furthest from the resolution of the story, when the lottery's ended but we don't know who's won the prize. During this act, we get to observe the interactions of the villagers during this ritual: we see Mr. Summers's efficiency in moving the lottery forward, Mrs. Hutchinson's eagerness to participate, Mrs. Delacroix's friendliness, Old Man Warner's crotchety resentment.
This section of the story is dedicated to showing us the village life that the lottery is designed to protect, as it brings these people together.
The third act is the resolution of the story. This is when we finally find out what exactly is wrong with the lottery, which has seemed only a bit suspicious up until now. The third act begins with paragraph forty-four, when it becomes clear that Bill Hutchinson has drawn the marked paper. Tess Hutchinson's distress at this is the clearest clue we've had so far that the lottery is bad.
We also see the worst betrayals in this section, as Tess tries to foist the drawing onto her own daughter, Bill doesn't try to protect his wife, and Mrs. Delacroix chooses an especially giant stone to throw at her friend.
As we discover what the lottery is really about, we see how twisted the tradition has made these villagers, that they are unable to protest the ritual murder of their neighbor, friend, wife, and mother, Tess Hutchinson. While Jackson spares us the gory details, she leaves us with no illusions that the "wrapping up" of this story consists in the stoning of one of its only rounded characters.