This is a great question, because the mood of the villagers is the eeriest aspect of the story, in my opinion.
First, the children gather. Jackson says that the children were boisterous. They just finished school, and there was a sense of liberty in the air. The mood of the children was one of excitement. Here is a quote that shows this point:
The children assembled first, of course. School was recently over for the summer, and the feeling of liberty sat uneasily on most of them; they tended to gather together quietly for a while before they broke into boisterous play.
The men gathered next. They seem nonchalant. They talked of planting, tractors, and presumably other mundane things. They also politely smiled and joked on the side. Here is a quote about this:
Soon the men began to gather. surveying their own children, speaking of planting and rain, tractors and taxes. They stood together, away from the pile of stones in the corner, and their jokes were quiet and they smiled rather than laughed.
Finally, the women came. They were exchanging gossip and trying to get their children.
They greeted one another and exchanged bits of gossip as they went to join their husbands. Soon the women, standing by their husbands...
As one can see, the mood is one of nonchalance, which is erie. They are about to engage in ritualistic murder, and they think nothing of it. It is just a part of their lives.