This episode is referred to as "The Lost Episode" because it was not included in the syndication package. It was not included 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 and was aired by CBS on December 18, 1989. In December 1990 CBS aired a colorized version of the episode.
This is one of two episodes specially colorized and combined, appearing as a one hour long 2013 Christmas Special on CBS, entitled the "I Love Lucy Christmas Special." The other being 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.