In The Giver, how do the people receive their assignment?

Assignments are given as the Elevens are about to become Twelves.  These assignments are the responsibility of the Elders, and they are announced by the Chief Elder at the Ceremony of Twelve.  As the Chief Elder notes in her speech, this is the only time that the community celebrates people's differences, rather than their Sameness.

Fortunately for the community, not all differences have been eliminated in people, since it takes different kinds of skills, talents,...

Assignments are given as the Elevens are about to become Twelves.  These assignments are the responsibility of the Elders, and they are announced by the Chief Elder at the Ceremony of Twelve.  As the Chief Elder notes in her speech, this is the only time that the community celebrates people's differences, rather than their Sameness.

Fortunately for the community, not all differences have been eliminated in people, since it takes different kinds of skills, talents, and interests to take care of all the needs in any community.  The Elders spend a year observing the Elevens, to see what their skills,  talents, and interests are. Since all of the Elevens have been engaged in a variety of activities, they can be observed in different kinds of settings.  Some are good at dealing with the elderly. Some are good at dealing with young children. Others might be good builders or show some aptitude in healthcare. Each child is assigned in a way that best uses his or her aptitudes for the good of the community. 

This probably seems like a good system to some readers, but a child who is twelve may have interests and aptitudes as yet unrevealed, and certainly, few twelve-year-olds have settled on one interest to pursue for the rest of life. But there is no thought of satisfying the needs and desires of the individual in this community, only the needs of the community.