What happens at the ceremony of one?
Since the society in the novel is an engineered society (not to mention a dystopian one), it is not surprising that this engineering begins at an early age.
When children are born they are cared for at the Nurturing Center until they reach one. When they reach this age, they are considered Ones. At the ceremony each child is given a name and more importantly placed in a family to grow. Presumably the child grows as typical children do in the family unit. Not much is said about childhoods in the book.
In one particular Ceremony of One, Jonas's father has Gabriel in his hand. Jonas is very happy about this, because this means that Gabriel has not been killed, which the community called being released.