In Chapter 16 of The Giver, what clues help you infer what the unnamed celebration is? When were you certain?

I'll put the clues in bold text:

Readers who celebrate Christmas themselves or who are very familiar with the holiday will recognize it right away in the memory delineated in Chapter 16, when we find out that it's the Giver's personal favorite, that it's full of joy, and that it takes place inside a warm room with "firelight glowing on a hearth." Then we see that there's snow outside the window. At this point, many readers already have grasped the atmosphere of a winter holiday, celebrated with a family gathered together.

A more obvious clue comes a moment later, when Jonas observes colored lights on a "twinkling tree" that is placed ("oddly," to Jonas) inside the room. Most readers know by this point that the memory is definitely about Christmas.

But if you weren't sure, you could keep reading and note that there's a smell of food cooking, and that there are brightly wrapped presents on the floor, and that the family members are opening the presents, laughing, and hugging each other. The image of the grandmother rocking her young grandson is especially touching and caps off the memory. Jonas never says the word "Christmas," and neither does the Giver, but it's absolutely certain that the memory was about that particular holiday.