Why does the Community wait until December to name their children in The Giver?

Children are not given a name for the first year of life, until they have met all of the requirements to join a family.

All children in the community are given a number when they are born. The number is, logically, based on their birth order.  Children are not born randomly in the community.  Fifty births a year are planned, and the babies are born to birthmothers and then whisked away to the Nurturing center, where they are cared for by a suitably distant staff until December.

Jonas's father's title was Nurturer. He and the other Nurturers were responsible for all the physical and emotional needs of every newchild during its earliest life. It was a very important job, Jonas knew, but it wasn't one that interested him much. (Ch. 1)

All children born in a year are officially the same age.  They may be physically different in age by a few months depending on when in the year they are born, but they are all still newchildren until December.  They are carefully tracked to make sure they are meeting the milestones for their age, and then assigned to a family unit. At that point, they are given a name.

When one of the newchildren does not develop as fast as he is supposed to, Jonas’s father looks up what his name will be.

I looked up number Thirty-six--that's the little guy I've been concerned about--because it occurred to me that it might enhance his nurturing if I could call him by a name. (Ch. 2)

Jonas’s father is actually breaking a rule.  No one calls the babies anything but their numbers for the first year of life.  They are nurtured, but dispassionately.  They are not loved.  Love does not exist.

This may seem strange to us.  After all, in our society we understand the crucial need to bond with an infant.  Those early months are very important to a child’s development, and they are also the months when babies get close to their parents.  This is a concept known as attachment.

In Jonas’s community, the desire for Sameness takes precedence over attachment.  Attachment is forbidden.  By taking the babies away from their mothers and having them raised in an institutional setting, the community pretty much guarantees that the babies will form less of an attachment when they do get assigned to a family.

Babies who do not meet the milestones by December, or who are deemed inappropriate for some other reason, are quietly released.

Release of newchildren was always sad, because they hadn't had a chance to enjoy life within the community yet. And they hadn't done anything wrong. (Ch. 1)

The population is tightly controlled and Sameness enforced by this procedure.  If no babies have been released, fifty will be given to families.  If a baby is released, it never has a family at all.

Having all of the children born in a year officially the same age, regardless of their actual age, and assigning them a name only when they turn “One” officially at the Ceremony of One in December ensures Sameness.  Everyone gets a baby the same way. Family units, which are created for the sole purpose of raising the children, only get two infants assigned to them (at different times).  Everything always happens in lockstep, to keep the community perfect.