How does the Ceremony of Twelve start and why is number Nineteen skipped in the assignments in The Giver?

The Ceremony of Twelve starts with a description of the class, and number Nineteen is skipped because Jonas has a special assignment.

The Ceremony of Twelve is the most important ceremony, and the last one.  After twelve, no one counts ages any more.  The children are well on their way to training to be adults.  The ceremony this year is different because Jonas’s number is skipped.  This causes him and the others discomfort, because they do not realize he has been chosen Receiver of Memory.

Jonas’s Ceremony of Twelve begins in an ordinary way.  The Chief Elder starts off by describing Jonas’s class of twelve-year-olds.

The speech was much the same each year: recollection of the time of childhood and the period of preparation, the coming responsibilities of adult life, the profound importance of Assignment, the seriousness of training to come. (Ch. 7)

After this introduction, the Chief Elder moves on to describing each child in number order.  The child’s personality is described, along with some brief history, and then the assignment is announced.  All proceeds normally until the elder moves from number Eighteen to number Twenty.

Jonas first assumes that he has done something wrong.  A child who does not complete volunteer hours is not given an assignment.  Jonas feels embarrassed and ashamed, and can barely concentrate for the rest of the ceremony as he is trying to decide what he did.

After all of the children other than Jonas have been given assignments, the Chief Elder finally explains what happened.  She apologizes for causing everyone confusion and apprehension, then tells them why Jonas was skipped.

"Jonas has not been assigned," she informed the crowd, and his heart sank.

Then she went on. "Jonas has been selected." (Ch. 8)

In a ceremony celebrating differences, the community acknowledges that Jonas is very different.  Standing out is never appreciated in the community, but Jonas has been given a special assignment.  As the Receiver of Memory, his position is one of great importance to the community.  It is an honor and very rare.

Jonas has no idea what he is getting into when he is chosen Receiver of Memory.  This position means that he will hold all of the community’s collective memories for generations back.  It will keep him apart from the others as he becomes one of the only people in the community to really understand what is going on there.

Why the drama?  Why skip Jonas at all?  The answer is that the community clearly wants to have everyone's complete attention for Jonas's announcement.  The position of Receiver of Memory is so important that they want to call attention to it, and to him.  It is an unusual move for a community dedicated to Sameness.  Jonas was definitely not expecting it.