Why did Jonas and The Giver choose the December ceremony as the time to implement their plan in The Giver?

Basically, Jonas chooses the Ceremony because it is the
one time everyone in the community can be guaranteed to be in one place. Symbolically,
however, the Ceremony is relevant because it is when Jonas’s life changed.  It was on
that day, during the Ceremony of Twelve, when Jonas became singled out.  From that point
on, he was never like anyone else in the community.  He also cemented his relationship
with The Giver from that time, and their relationship is built on shared pain. 
Eventually, trust is added to this.  Jonas’s journey began then, and by taking Gabriel
when he was supposed to enter his own ceremony, Jonas transforms him
too.