I would describe the atmosphere at the beginning of the story as pleasant and lighthearted. The community of a small New England village is assembling on the village green on a June day for a lottery. The weather is fresh and warm, flowers are in bloom, and the grass is very green. Children have been let out of school, so they are in a festive mood, while men and women gather in groups, softly talking and gossiping. The lottery is likened to such events as square dances and the teen clubs, because Mr. Summers runs them all. We have no reason to expect that the lottery will be anything but the most cheery of experiences, and it is easy to miss the ominous notes that Jackson plants, such as the pile of rocks the boys gather.
Ominous notes begin to pile up, however, as the story proceeds. The splintered look of the black lottery box which nobody wants to repair is a little eerie, as is the "hush" that falls over the crowd as the lottery begins. It is also unsettling when both Mr. and Mrs. Adams note that other villages have dropped their lotteries, as if that is a good thing to do. Old Man Warner's warning that the lottery is needed to ensure a good harvest also acts as a hint that this is not a normal lottery where someone simply wins a prize.
Everything turns very dark when Tessie Hutchinson cries out that the lottery is not fair:
Suddenly, Tessie Hutchinson shouted to Mr. Summers. "You didn't give him time enough to take any paper he wanted. I saw you. It wasn't fair!"
At this point, it is clear this is a high-stakes lottery in which nobody wants to pull the "winning" ticket.