What is the Ceremony of Twelve, and why is this the most important ceremony in The Giver?

The Ceremony of Twelve is the most important ceremony in Jonas's community because it is when the adolescent children are given their permanent Assignments, which are their individually selected occupations. It is the last ceremony each year and is considered a rite of passage to becoming an adult.
The Committee of Elders carefully monitors each adolescent leading up to the Ceremony of Twelve as the children volunteer for different occupations. The elders then choose the best Assignment for each child based on their personalities, interests, and abilities. Leading up to the ceremony, Jonas is nervous about what his Assignment will be.
He is initially skipped when the Chief Elder begins presenting each adolescent their future occupation. After each child receives their Assignment, the Chief Elder informs the community that Jonas has been selected to be the community's next Receiver of Memory, which is the most prestigious and important job in the entire community.

The Ceremony of Twelve takes place each year in December; at this time the Elders select the position that each Twelve will hold in the community. Since this is the final ceremony, it represents a rite of passage into adulthood and is, therefore, the most important ceremony performed for members of the community.
No other ceremonies are performed after the Ceremony of Twelve. Also, this is the occasion on which differences are acknowledged. The Elder tells the soon-to-be Twelves:

"You Elevens have spent all your years till now learning to fit in, ....But, today we honor your differences. They have determined your futures. (Ch.7)

The new Twelves are called one-by-one according to their birth order; then they are given their Assignments, their roles in the community which they will perform for the rest of their lives. The Twelves are assigned roles in the community that the Elders have chosen for them, based upon their observations of each child. After this ceremony age is no longer important, and birthdays are no longer celebrated.