The episode is from the show's eighth season- from 1981- and involves the entire extended Ingalls family. In it, family friend Hester Sue Terhune arrives for Christmas Eve dinner at the Little House, bringing with her Mary and Adam, who were not expected. Laura and Almanzo are also there, and as they eat supper the blizzard which has been threatening hits hard, making it impossible for the guests to leave. As the children head off to bed, the adults sit around the table, telling stories of Christmases past. The set up is that of a clip show, but actually only one of the stories utilizes old footage from the show. The others were filmed for this episode.
Those are my thoughts on The Christmas They Never Forgot: a so-so episode which is serviceable, but could have been a lot better. And I think Almanzo's story is terrible. As a side note, this was only a regular length episode, so we watched another one afterwards- a much superior episode from the fifth season entitled Mortal Mission. I'll review that sometime after Christmas.