Assuming that the book's climax is when Jonas realizes what to be "released" really means, main events occurring to that point would include:
1. Father brings Gabriel home to help the sensitive baby adjust to life in the community so he won't have to be "released".
2. The December Ceremonies take place; they are especially significant this year because, as a Twelve, Jonas will receive "assignment" to his adult role in the community.
3. Jonas is passed over for regular assignments, and instead is selected to become the next "Receiver of Memories" because of his special gift to "see beyond".
4. Jonas begins training with the "Giver". He is fearful because he does not know what his job will entail; a new "Receiver" is not often selected, and the last one failed in her assignment.
5. Jonas is given memories that are withheld from the general community; they begin pleasurably with ones such as "sun" and "snow", but progress to include the horrific, such as "hate" and "war".
6. Jonas's training causes him to begin questioning the insulated life of the community.
7. Jonas sees a tape showing his father coldbloodedly administering a lethal injection to a baby who is being "released", and realizes the monstrosity behind the euphemism. He understands the fallacies underlying the society, and in particular, what it means for Gabriel.