What evening ritual did the family perform after dinner?

The answer to this question can be found near the beginning of chapter one.  After dinner, Jonas and his family begin the nightly ritual of telling each other the feelings that were felt over the course of the day. 

It was one of the rituals, the evening telling of feelings. Sometimes Jonas and his sister, Lily, argued over turns, over who would get to go first. Their parents, of course, were part of the ritual; they, too, told their feelings each evening.

The ritual could be viewed as a device to monitor the mental and emotional health of each society member, but at this point in the story, readers are unlikely to have this suspicion.  The ritual comes across to readers as perhaps a bit strange, but it does give the parents an opportunity to have teachable moments with their kids about the emotions moving through them. 

"How did you feel when you were there?"
Lily frowned. "I felt strange. Because their methods were different. They were learning usages that my group hadn't learned yet, so we felt stupid."
Father was listening with interest. "I'm thinking, Lily," he said, "about the boy who didn't obey the rules today. Do you think it's possible that he felt strange and stupid, being in a new place with rules that he didn't know about?"
Lily pondered that. "Yes," she said, finally. "I feel a little sorry for him," Jonas said, "even though I don't even know him. I feel sorry for anyone who is in a place where he feels strange and stupid."
"How do you feel now, Lily?" Father asked. "Still angry?"
"I guess not," Lily decided.

Jonas shares his feelings last, and he tells his family that he feels "apprehensive."  He feels this way because the Ceremony of the Twelve is coming up.  Readers have no idea what it involves, but the fact that Jonas has this particular feeling about it alerts readers to a possible future conflict.