Christmastime with Mister Rogers was a hour-long holiday special of Mister Rogers' Neighborhood that aired in December 1977. It was produced between a hiatus that extended from episode 1460 in 1976 and 1961 in 1979.
What's odd is that quite a few articles exist on it, including a recent one to celebrate the episode's 40th anniversary, but no one seems to know where to find it. It makes sense since PBS specials from the 70s are notoriously difficult to come across, but still odd. Especially considering that the Fred Rogers Company almost definitely still has it and it doesn't have anything controversial like the Conflict series that would prevent it from airing. I checked and am almost certain it didn't air as part of the twitch stream of every episode this past year.
Maybe I'm just being stupid and it's somewhere I'm not searching, but if not this might be one of the last pieces of lost media from that show ahead of its 50th anniversary.
Petition to rename the Lost Media Wiki into the random shows I remember from Treehouse TV forum