This episode is referred to as "The Lost Episode" because it was not included in the syndication package, because of its Christmas theme and because it mostly consists of flashbacks to previous episodes. This episode remained unseen until the year Lucille Ball passed away; CBS aired it on December 18, 1989. In December 1990 CBS aired a colorized version of the episode.
This is the first-ever clip show/"flashback" show on television.
This is one of two episodes specially colorized and combined, appearing as an hour-long 2013 Christmas Special on CBS, titled the "I Love Lucy Christmas Special." The other was Season 5, Episode 23: "Lucy's Italian Movie."
One reason that this episode of clips from past shows was put together was that many people in the USA did not have television sets when "I Love Lucy" first came on the air in 1951. TV sales had skyrocketed in the last 5 years--as had the number of "I Love Lucy" fans--and this "retrospective" episode gave many people a chance to see clips from previous shows for the first time.
The "flashback" clips are taken from episodes 50, 52, and 56.