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.
Jonas and the Giver choose December as the time to implement the escape plan because it coincides with the community's major ceremony of the year. Everyone from the community must attend with the exception of the Giver. The event provides the perfect opportunity for Jonas to gain some time to escape without detection. By the time, Jonas' absence is noticed, he and the Giver hope Jonas already will out of the community and on his way to Elsewhere.