In Chapter 16 of the book, The Giver has just show Jonas a memory of war. It was terrible and Jonas was really shaken. So now The Giver is going to show Jonas a good memory. What he ends up showing Jonas is a memory of a Christmas celebration.
In the celebration, we see an extended family having Christmas together. This seems really weird to Jonas, but he likes it because he feels so much more love in that memory than exists in his own life (or in anyone else's life in his society).
I'm not quite sure what you mean by what has "happened to" the original event. Christmas and family stuff like that has been wiped out by the government as a way to make people more dependent on the government and less independent.
In chapter 16 Jonas sees a tree with colored lights inside a room. In the room are two parents, children, and two older people. He comes to realize they are grandparents. A child is passing around gifts and children are opening them. The feelings evoke warmth, family, and love. The gift is the memory of Christmas.
Once the memory has been transferred to Jonas it is lost from the Giver. He no longer has the memory. The memory was probably his from his childhood.
Grandparents in Jonas' society do not exist. The older parents are sent to live together away from their raised children. There are no sets of generations in the home other than the children given to the parental unit and the parental units.
Christmas, like grandparents, has been terminated by the society in which Jonas lives.