Why did Jonas and The Giver choose the December Ceremony as the time to implement their plan?This answer is in Chapter 20.

Jonas and the Giver chose this time because everyone would be so involved with the Ceremony that they most likely would not notice Jonas's absence for awhile, giving him time to get away from the community and be on his way in his search for Elsewhere.  It also would allow the Giver, who would need to help Jonas in the initial stages of his escape, a chance to be absent for a time without suspicion, because he ordinarily did not attend the December Ceremonies.  By the time Jonas's disappearance would become evident, the proceedings would be well under way, and although a few searchers would be sent out after him, most of the community would be otherwise occupied with the Ceremonies, which could not be disrupted under any circumstances.  According to their plan, once Jonas had successfully gotten away from the community, the Giver would return to the Auditorium where the people would still be gathered after the Ceremonies; he would announce that Jonas had been lost in the river, and would begin the Ceremony of Loss.  After that, the people would be consumed with the enormous task of bearing the memories Jonas had acquired on their own (Chapter 20).