Why does the day of the Ceremony of Twelve seem like an ideal time for Jonas to leave the community in The Giver?
The Ceremony of Twelve represents one of the few times when all members of the community are gathered together in the same place at the same time. Some are there because they are the children being recognized, advanced into the next year of their youth and the new symbols of age and the new responsibilities that are assigned for that year. Some are there as parents of the children, involved in the raising and indoctrination into the life patterns expected by the community. Many are there because the community functions as a whole - all are impacted by anything that changes the status of any part of the group.
With all of the community attending the Ceremony of Twelve, Jonas realized this was his opportunity to depart without being noticed or missed. Since leaving the community was unthinkable in the usual pattern of actions, it was vital that he was not seen as he left. All attention would be focused on the celebrations connected to the ceremonies for the different ages, an all-day proceeding that would give him plenty of time to travel far away from the community before he was missed.